You should be refering to ATI Radeon 9600/9800, right?
I'm using a x1300 (the veeeery lower end of the new cards, and to be worse it's AGP not PCI-E) and SRO runs smothly with 1208x1024 resolution with all settings turned on, except detailed shadows (with it on, it remais playable, but somewhat choppy).
For the newer cards (Geforce FX and up, Radeon 9600 and up) it's really a matter of preference, both brands perform nearly as good.
When I next update my card, I'll probably get a Geforce 6800 or a 7X00. Mainly because NVidia still makes AGP cards, and I don't feel like buying another board and processor soon. (I'm running on a AMD64 3200+ socket 754)