Its matter of comparing sky(7900 GTX 512) with earth (X1900 GT 256)
No way ... ATi has a far more efficient memory controller , infact the 512 bit mem. controller ensures lower latencies , so u can expect really efficient mem. operations . Bundle that with a high core speed , u already have a very powerful card even with 256 MB Vram ...
As for the X1900GT , infact when oc'ed can touch oc'ed 7900GT's .. the only plus for the later is that when volmodded they give insane oc's ...
Anyway , its not about dominance here , just technical discussions ..