That looks like a perfect XP system.

You could always make it a bit more high end, hot and power hungry but I'm not sure it would be an improvement...

For example a HD4870 1024MB or HD4890 is a bit more period correct and slightly faster than the HD5770 but they also have double the power draw.

The Core 2 Extreme QX6700 is only $25 or so and is slightly faster than the Q6600, but again uses much more power for a 5% real world performance increase... The Q9650 costs $60 for a 15% performance increase.

The best thing I can think of for improving your system is to get a better motherboard like a high end P35 or X38 board and a decent CPU cooler so you can run your Q6600 at ~3.2 GHz with stock voltage, perhaps this is even possible with your current motherboard.

You can also get a Sound Blaster Audigy 2 or Sound Blaster X-Fi if you use the onboard audio.

If you actually want to play current games then get a new computer and use the old one as dedicated XP machine.

1) Overclock Q6600 to 3GHz (FSB 1333MHz is supported on P31, with 1600MHz "available" through manual OC)
2) Forget about DX10 stuff (unless you are interested in "period correctness"), you have a great card for XP games.
3) Xeon E5440/E5450 "pre cut for LGA 775" + LGA 771 BIOS mod.
Those Xeons may not overclock on your board, but they will use less power than Q6600 and be faster in default settings (assuming you won't/can't overclock Q6600).

The most modern GPUs with XP support are the AMD HD7800 series and a handful R5/R7/R9 cards that are rebranded HD 7800 chips. For Nvidia it is oddly specific, the last GPU with XP support is the GTX 960, the 970 or higher don't support XP, the 960 does for whatever reason.

What agent_x007 proposed. About the Xeon you can also settle for a modded X5460/5470 if you don't care about power consumption. you can find these for $15 and they are practically equal to a QX-9650 or a first gen i5. You would need an aftermarket cooler for these chips (like an Arctic Freezer 7 pro) cause they can get quite hot, especially in the summer . But E5450 imo is the best choice with a much lower tdp.
Pair it with a cheap but fairly modern gpu like the GTX750ti or HD7850 and you have a very capable directx11 gaming system. With the dual boot you could play almost every game that came out over the last two decades. Nvidia and amd has drivers for win xp for both cards I mentioned.

Personally I wouldn't bother with the cpu unless it was to reduce power consumption but then again tinkering with the volts along with a small overclock it would do very nicely, os wise I would just run 7 given that it is a quad core system though a there is a trickle of interest in vista lately and it is period correct. As for the graphics card you don't need to waste a heap on a modern GTX 960 when there are so many choices on the cheap that will upgrade over a single 5770.

I would suggest a Hd7850 or HD7870

Thanks all for the advice.

I overclocked the CPU to 3.0 GHz and it boots just fine. I didn't change the voltage.
I played Witcher 1 for several minutes, no problem. But when I ran the "stress" test under CPU-Z, the mainboard started buzzing after a short while. When I clicked Stop, it stopped.
Perhaps the stock cooler isn't enough.

I'll consider buying Radeon HD78xx though I'm a bit worried that they have considerably higher TDP than 5770.

There is a reason why I suggested tinkering with the voltage to make it a little lower, a lot of meh to low quality boards have been ruined over the years because the vrms couldn't hold up to a overlcocked q6600 and sadly some good boards as well. I managed just under 3.2ghz on a Q8200 with less than stock volts years back.

HD7850 came today and it's better than I though it will be: now I can play Tomb Raider 2013 on Ultra in 1600x1200 or 1680x1050 with 55 fps. HD5770 had 30 fps in 1280x960.

Oddly, I can't seem to be able to disable GPU scaling, which means 800x600 is blurry on my 1600x1200 monitor. Some people report having the same issue with some driver versions, I'll have to investigate this.

The new card is much quieter in games, but a bit louder in desktop. Seems that it has less variation in fan speed.

when I had a g31m-s2l it couldn't really handle a 65nm C2Q @ 3GHz, it was stable for normal usage and most regular tests, but Prime95 would shutdown the PC; the power load is to high I think,
you could replace it with a 45nm quad core, even one of those moded Xeons like the E5450 for example;
the 7850 is a very nice card, it can handle even most current games (at lower settings), but if you try to play newer games 4GB of ram becomes a limitation.

To be blunt as a Q6600 owner, I'm not sure it's cost-effective to upgrade it. Haswell leaves the old Kentsfield chips in the dust even when you're dealing with roughly GTX 480-caliber cards, and Skylake/Kaby Lake/Coffee Lake have only increased the gap.

Heck, I have a P35-DS3P 2.0 that I can't really hit 3.6 GHz with, even on water with a G0 stepping; it'll POST, it'll run and take on IntelBurnTest and all that, but it actually performs a bit slower until I dial it back to the 3.0/3.2 GHz range. Maybe the VRMs aren't up to snuff with all the Vcore I'm shunting into that thing?

If anything, I would've kept it as is and just dedicated it to XP use, since you're only on 4 GB of RAM and won't be losing as much of that running a 32-bit OS (nobody runs 64-bit XP anyway). My Q6600 build is maxed out at 8 GB, so I couldn't bring myself to do that. It's also pretty much overkill for any XP-era game; just add an X-Fi card for hardware DS3D/OpenAL and you're set.

Id drop a cherp 45nm quad and call it a day. The video card is a good one and unless if your reqlly pushing the cpu with an overclock or plan to replace the board and cpu its not worth upgrading most likely.

Edit. I use xp64 over 32bit when ever possible.

NamelessPlayer wrote:

If anything, I would've kept it as is and just dedicated it to XP use, since you're only on 4 GB of RAM and won't be losing as much of that running a 32-bit OS

I use a hack (patchpae) that unlocks access to full 4 GB (or more). But as far as I know, 4 GB is the max for this mainboard.

just add an X-Fi card for hardware DS3D/OpenAL and you're set.

What are the advantages?

Azarien wrote:

I use a hack (patchpae) that unlocks access to full 4 GB (or more). But as far as I know, 4 GB is the max for this mainboard. […]
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NamelessPlayer wrote:

If anything, I would've kept it as is and just dedicated it to XP use, since you're only on 4 GB of RAM and won't be losing as much of that running a 32-bit OS

I use a hack (patchpae) that unlocks access to full 4 GB (or more). But as far as I know, 4 GB is the max for this mainboard.

just add an X-Fi card for hardware DS3D/OpenAL and you're set.

What are the advantages?

I've never heard of patchpae before, but I should look into it. After all, the time I made the jump to the Q6600 was also the time I jumped to 64-bit Vista alongside it.

As for the sound card advantages? Most of it just involves better sound in general from games designed to take advantage of an Audigy or X-Fi, because they don't sound right on software OpenAL implementations - not even the one Creative uses on their newer Sound Core3D cards.

The other big reason, if you use headphones instead of speakers, is CMSS-3D Headphone. It's the closest you're going to get to Aureal A3D-class positioning over headphones on a post-Win9x build.