ATI's magic goal is to achieve 400MHz/800 MHz for the card and memory while we know that it can easily clock it to 375 MHz, which can be considered as the lowest frequency of R350.
Performance of the chip still has to be seen but I suppose ATI's target is around 10-20% above that of GeForceFX's.
Now I can tell you, if they accomplish that and don't arrive too late to the market, they would be easily reassuring their current leader position at least for the next 8 months; NVIDIA, in the other hand expects ATI to have more than one problem moving to .13 process.