Yes you guys are right, sorry. Didn't think of the big RED machine.
Yes a 5850 will do you good.
ATI really handed it to nvidia this year. Their 5890 chip is 1500 shader pipelines,, the Fermi has 512 shader pipelines. This is according to me doing research on the web. Things can change..
They aren't directly comparable, nVidia's stream processors are more encapsulated and fat and are properly designed for linear scaling performance with little driver dependency and predictable performance, which means that each stream processor can execute one instruction at a time (Fermi will change that). ATi's HD 5870 stream processors consist of a 320 clusters of stream processors and each cluster have 4 mini alu's plus 1 fat alu in each one of them which requires clever compiler optimizations to maximize its execution resources.
It means that for example, in the worst case scenario which is mostly never, only 1 stream processor of each cluster will work and the other 4 remaining will idle. In the best case scenario which is challenging to get there but near to 85% of utilization, the 5 alu's will work together as far as they're working together with the same thread.
In resume, ATi's execution resources are wider than nVidia's architecture (Of course current GT200 architecture), fermi isn't much wider, but more efficient hopefully.