According to FUD Zilla the upcoming R600 GPU will have a total bandwidth of 102.4GB/s with its 512 bit memory interface and 1600MHz GDDR3 memory. This is almost 20 percent more than the NVIDIA GeForce 8800's 86.4GB/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.