First, let me say hello to everyone on here, I've been using this site for a while now to look for information regarding old configurations and setups, but now I've finally registered because I need some help with a pet project of mine.

The thing in question is a (like the title says) socket 775 based prebuilt by a German company Medion, namely model 8818.

I've had this computer since new in late 2006 and was using it as my primary one for over 10 years (yes, I know, 🤣), but for a while now, I've had this (totally irrational, of course, but hey) idea to fully max out its specs as far as they would go (or at least as far as I could get parts). Now, I've already done a lot of digging around for info and even accumulating different parts, but as it turns out, this one is quite a bit of a doozy and there is no definite collection of information on it, especially not reliable.

To begin with, the computer first came with:
MSI motherboard 7318 with a VIA 890 chipset (BTX form factor)
core 2 duo 6300
2x512Mb of ddr2 ram (single channel only)
Nvidia 7650gs

Now, the gpu and the ram I relatively quickly upgraded (like 6 months after purchase) to 7950gt (150€ on sale!) and 2x1gb. I actually wanted to buy and install the (at the time) new 2900XT, but quickly realized (fortunately in retrospect, I guess) that I would have to also upgrade the PSU and, more importantly (and that comes into play later), that the case can only take single slot cards, or at least, they can only have a single slot bracket, because of some motherboard ports right next to it. Further back, the card can be wider (but no too much) and also not too long.

So, the computer stayed the same for a long time, until the gpu started artifacting and I replaced it with a cheap, second hand HD 5570, which despite being a small card, barely fitted - like 2mm wider and it would hit cpu cooler mounts, 3mm longer and there is a capacitor. Some time after that, the PC was finally relegated to backup service and that's where I first got the idea to at least moderately upgrade it and keep using it as a Win xp era gaming build (that's also the OS it came with). But, since it was the height of (previous) mining craze and everything newer than 500 series on the Nvidia side and 6000 on the amd was stupidely overpriced, I first started with the cpu and ram.

I very quickly found out that the info on the computer/motherboard (really one and the same, since it was a custom one just for this model) was at best incomplete - according to one source the most it would take is 2gb of ram, yet it happily works with 4, according to another, it only supports dual core Conroes, yet Q6600 works fine (although really stressing the cooler, which is completely custom and so are the holes in the board or rather their spacing).

So, when the craze was over in late 2018, I pretty much decided on the final specs it would get - E6700, 4gb of ram and 750Ti which I got for 40€, but here's the first real catch - the card gave no post, no matter what, yet worked just fine elsewhere.

This bummed me out quite a lot obviously, but during the next months I slowly got my hands on several different gpus and what I came to realize was that the board will not work with anything pcie 3.0 or newer. 6000 series radeons work, 7000 and up don't, likewise for 500 series Nvidias and ... well there is a gap - I haven't yet tested a 600 series card, only a 700 and up, or rather I did but it turns out, it was a broken one (gtx 660). But that realization was recent and so I thought the best gpu that could work with it, was a gtx 580.

Obviously, there were no single slot 580s, however their default output config (2x dvi, 1x mini-hdmi) makes them single slot ... if using a waterblock. Now, buying a gtx 580 + waterblock + rad + pump + reservoir + tubing certainly doesn't seem like a worthwhile purchase for something that, let's face it, could never properly utilize that kind of power in most scenarios...however, I just happened to score a mint 580 block from EK for 10€, then not much later, not one, but two Zalman Reserators complete with some tubing and extra cpu blocks for 45€, 🤣. All that was left then was the card itself (had to be one with reference PCB at that point) which I also got at a reasonable price and the path seemed set. I had both the fastest regular dual core (E6700) and quad core (Q6700) for it (extreme editions proved...difficult to obtain to say the least), max ram, ssd and what I thought was the fastest single chip graphics card.

However, due to several reasons the project got shelved just before completion basically and when I recently decided to finally finish it, I came across this:
https://www.userbenchmark.com/System/Medionpc-MS-7318/6892
Two users with this motherboard in the last year and a half seem to have posted userbenchmark scores with a gtx 650! If it was only one, I would probably dismiss it, especially after having spent so much time and effort (not really that much money, luckily) tracking down a gtx 580 + watercooling, but if this is true, it probably means, there are at least a couple gtx 680 models that might work with it and they would obviously be considerably faster than a 580, no matter how far I would push it under water.

And here is my main question to anyone familiar with this computer/motherboard (admittedly not likely with so much newer gpus) or at least the chipset - what is going on here?
I've tested literally dozens of cards with it (latest bios, obviously) and consistently, those with pcie up to 2.1 worked, while those with 3.0 didn't.
You might say, none of the newer ones could work with a mobo with legacy bios, but at least two or three were supposed to (not to mention I have a gtx 980ti that happily works on a Asus 975x board), so how could those guys get gtx 650s to work?

If I had an access to a (working this time) 600 series card, I would obviously try it out immediately, alas, none of my friends have one, nor is any sold locally (at least not close enough for me to get there and test it) and current situation has driven even the prices of nearly a decade old cards up, admittedly not as much as newer ones, but enough for me not to want to just buy something online that might not work in the end. Also, due to the aforementioned space limitations, I did the testing with a help of a "mining" x1 to x16 riser cable, not a full x16 to x16 one, but all the older cards would run fine, so I can't quite see the issue there.

Oh, as I've pretty much said already, regarding the best cpu, the issue is much clearer - the mobo seems to only work with 65nm core 2 series at the latest and with fsb of at most 1066Mhz, which means the theoretically fastest cpus would be X6800 for the dual cores (or the super rare, never officially released X6900, 🤣) and the QX6800 for the quad cores (however, I don't think the vrms would be up to the task with the latter). Now, the bios is extremely locked down, like you pretty much can't change anything but the clock (not kidding), so any overclocking is out of the picture, I did however try a so called BSEL mod on a 800 fsb E4600 which should bring it to 1066 and therefore 3.2 Ghz, but with no success. The cpu still worked fine, however nothing got changed (weird in itself because as far as I could find, if the Bsel mod doesn't work, you often don't even get a post). So, if anyone has any input on that, It'd be much appreciated as well.

I know this is a (very) long and winding post, but it's been (already) a long project and I feel that by condensing it too much, I might prevent someone seeing a solution. Thanks in advance for any useful advice.