I doubt you'd notice any difference between the two GT's. The GTX is faster. They are all manufacturers at the cheaper end of the market, and here they tend to stick very closely to the Nvidia reference design.
Having said that, the 7800gt is a beast of a card; is the rest of the system able to do it justice? What kind of processor and ram do you have? What games do you want to play? Will your power supply be man enough for the card?