thinking of upgrading the processor to a Intel Core 2 Extreme QX9650 but prices are weird on ebay etc and most of the cheapest ones seem to come from china and not sure if to go for that.
I'll probably be the only one to "latch onto" this point. Have you considered getting the exact same processor as the Q9650 quad-core, but using the Xeon E5450 instead and doing a simple mod? The thing is, the E5450 Xeon runs at 80 Watts TDP where the Q9650 is 95 watts, and the QX9650 runs 130 watts, so it runs A LOT cooler. They also have different thermal limits - the Q9650 has the highest thermal limit before throttling (71.4°C), the Xeon is in the middle at 67°C, with the QX9650 the lowest at 64.5°C. They're both 3.0 GHz stock and they both have the same cache layout, same FSB speed, same core count.
I bought a Q9650 for around $45 US + shipping and then bought TWO E5450 Xeons for about $20 US each + shipping, so effectively I got two Xeons for the same cost as the Q9650. One is running my home media server and the other is sitting on my desk waiting for a new LGA775 project.
I carefully used a Dremel tool to put new notches into the CPU package substrate in order to fit into a standard LGA775 socket and then put the little pin sticker on. I've tested both and they both work just fine on an LGA775 motherboard. The only tricky part is if you want to mod your BIOS to include the microcode for those CPUs, which I did successfully on two motherboards. The DELL bios was tricky only because I had to go through a number of steps just to unpack the BIOS files in the first place (they were packed into an EXE that I had to figure out how to unpack just to get to the files I needed to edit). The editing itself was quite simple.
In fact, I think Phil's Computer Lab has done a video on this very mod as well if you want to have a look on YouTube. Though I think a quad-core CPU is a bit over-kill for Windows XP, a dual-core would be more than powerful enough. I ran Vista on a Core 2 Duo for years with no issues, though I never did try to play games on it - it was only a daily use computer for basic office tasks like web surfing, video playing, and some minor games like mahjongg and solitaire.
But I'll be following this thread, because I have an LGA775 system with a Pentium dual-core I was thinking about getting a gaming graphics card for to see how the Pentium measures up to a full-blown Core 2 Duo in Windows XP for gaming. The big difference is it only has 1 MB of L2 cache, where the Core 2 usually has double or more. The Pentium I think is around 2.6 GHz, so it's already way more than the 2.1 Pentium M single-core on my 2005 gaming laptop, and that thing was a gaming beast back then (7800GTX graphics chip with 256MB video memory).