I've played Doom3 on an Athlon XP 2600 NForce 2 system with 1 gig of ram and a GFFX 5900 flashed to 5950. I also played Doom3 on an Athlon64 3200+ NForce 4 sytem with 1 gig of ram and a 6600GT. I could'nt really tell a difference visually playing each at 1024 x 768, high settings.
It seems quite a bit faster and smoother on the 6600GT, but it was very playable on the 5950 also. My framerates mostly were in the 40s and 50s range with the 5950. Im sure the 1 gig of Corsair XMS PC3200 helped that quite a bit