The top dog for anyone wanting to go the Nvidia route is the 8800 Ultra, it’s a fast card but it doesn’t exactly scream performance-to-cost ratio. With pricing that starts around $600US to $700US, they’re certainly not cheap.
The HD 2900 XT is the card sitting up top of the ATI pyramid. Pricing is closer to the $400US mark for just about all brands. A pair in crossfire is going to set you back around $800US.