7900 rocks the 7800. About 10 gb/s more memory bandwidth, 4 more pixel pipelines i believe, faster memory by a lot, the core speed is faster on the respective models from each line, ahhh..... smaller die? 90 nm? not sure if the 7800 was on that too... anyways i would get the exact specs but im running out the door
good luck, but definately choose the 7900 gt