I've seen lots of benchmarks for GPUs using Dx10/Dx11 and W7, but I don't have a clue about how those translate to Dx9 and XP.
This is my current setup:
E6750 @ 2666 Mhz (333x8) (Supports x6/x8 multiplier)
2GB DDR2 Kingston @ 666Mhz (333x2)
Sapphire Radeon X1950 Pro 512Mb
Windows XP 32
Two monitors, both DVI, one 1920x1080 and the other 1280x1024
I mainly wanted to replace the GPU so I can run some sims (like Lock On: Flaming Cliffs, FSX), but also the newer RTSs (Shogun 2, League of legends if you could call it that) or PCRPGs (Fable 3 if it supports Dx9, The Witcher 2). Most of them ran fine at 1280x1024 in my current setup, but with my new 1920x1080 monitor I had to turn off shadows or other settings for them to run at >50 FPS.
Another upgrade I could do to the computer would be buying a set of DDR3 memory, and set it at 900Mhz(450x2) and the CPU at 6 multiplier (450x6, 2700Mhz) if that's needed to take the most of the GPU that would cap my setup. I may also consider switching to W7 either 32 or 64 bits if you think my computer would make it worth.