Do you want to do SLI or Xfire ?
If not then what is your planned use case ?
I have a lot of 775 boards and just as many CPUs, if you want a nice easy Core2 Quad system then avoid the top end stuff and find a nice P45 board, Gigabyte boards are a plenty on eBay but I wouldnt go looking for one of the DDR3 models, a nice GA-EP45-UD3P would be a very nice solid board that wont break the bank and it also supports XFire, grab a pair of HD5850s, 4gb of DDR2 1066 memory and a X-Fi Sound Card and itll make for a very nice XP rig. If you want SLI then things get a little more complicated as you will want a SLI board unless you are willing to jump into the filthy world of hacked SLI drivers.
But for SLI a EVGA nForce 750i/780i motherboard with the above parts but two GTX280 cards will make for a good solid SLI rig, going SLI will actually cost more as both the motherboards and GPUs have the nVidia tax on them and from my experience dont make for nice budget 775 machines, but they are exceptionally powerful machines if you are willing to throw cash at them. I have a QX9770 SLI GTX295 rig and its a monster that well .. cost far more than I would have liked 🤣, the motherboard was the hardest bit to get as ASUS Striker II extreme boards are rather rare on eBay. (I dont recommend a build like this)
If you dont want either SLI or Xfire then things get a bit cheaper as you can just use the above GA-EP45-UD3P with whatever brand GPU you like and itll work wonderfully with either CPU you choose, I still recommend 4gb of 1066 ram as there is no reason to not use the full amount XP32 can address, of course with 4gb you could even throw Windows 7 on it as dual boot.
If you are wondering why I went with Gigabyte here its because Gigabyte has been great at leaving up all their legacy drivers and bios updates which makes it super easy to find them but if you have other preferences then make sure you can get the latest drivers for the boards before you buy it, its a pain in the ass trying to setup a new system only to find you now have to spend hours hunting down drivers because the fab took them down.
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Not sure if you agree but the GA-EP45-UD3P is a great looking board while having a huge amount of IO and PCIe slots.