Well, if you don't like ATI, you're missing out, the 4850 is the best deal out there (maybe 2 4670's for budget Crossfire as well). For Nvidia, I'd consider the aging, but still good 9800gx2 or a GTX260. I don't think the 9800gtx brings enough to the table in this generation.
Always check to make sure you have enough amps on the 12 volt rail for any high end card.
Also, CPU limitations disappear a bit at higher resolutions. When a budget CPU (like my aging X2) is matched with a high end card at 1280x1024, then the CPU can't keep up with the number of frames the GPU can deliver, so there's a bottleneck. That can happen at higher resolutions, I recall a review where a Q6600 couldn't keep up with a 4870x2 until the resolution past 24" LCD. I don't recall whether it was overclocked or not, or what motherboard was used. I'll look for it later this weekend.
Saturday morning, I'll order an 8750 and put it on a Gigabyte 780G board and see if the bottleneck disappears. Before I upgrade, I'll test with all the benchmarks I can and then repeat after. Still have a 17" Viewsonic CRT, but I'm looking at either a 22" or 24" LCD right after Thanksgiving.