EVGA has a 7900 GT "signature" clocked at 600mhz core and 1600mhz memory for $399 for a limited time. If you OC the core to 650mhz you have your self a 7900 GTX. Not sure how much OCing headroom is left in it though but eVGA markets it as "600+ MHz".
So for 100 dollars cheaper you may
get yourself a 7900 GTX. Even though you can get a X1900 XT for 40-60 dollars more. But another thing, is that single slot cooler enough for a card clocked at 650/1600?