RequiemSchism:GTX280 is the best Nvidia, but 2 4870x2s will out perform 3 280s in almost all applications. 4gigs of GDDR5 vs. 3 gigs of GDDR3. The 280 GPUs bring it up to be a close competitor, but the GPU/Vram just barely gives an advantage to ATI. But if you're set on an nVidia system, 280s are the way to go.
For this to even matter though he'd have to be playing at resolutions of 2560x1600 or even higher! I wonder when we'll start seeing even higher resolutions?
Coolermaster Centurion 5 + Coolermaster Real Power Pro 750W
Gigabyte GA-P35-DS3L + Intel Pentium E5200 @ 280FSBx12.5
2x1GB G.Skill DDR2-800 @ 840 4-4-4-11 1T + XFX 8800GS XXX