Radeon HD 2900 XT has much better memory bandwidth then any card before including G80GTX. With it
512 bit memory interface and 1600 MHz GDDR3 memory it can score 102.4 GB/s.
Nvidia's G80 with its odd 384 bit memory controller scores 86.4 GB/s.
The new R600XTX with GDDR 4 clocked at 2200 MHz as originally planned can score the cunning 140.8 GB/s. Both cards have twice or close to twice more memory bandwidth from its Radeon X1950XTX card. This card was limited to now modest 64GB/s.
So with 102.4 GB/second you wont get much performance penalty at least on some FSAA settings and lower resolutions. The R600XT can work out 518GFlops and that is only with the Shader units.