The 7900GS is an excellent card - it gives just slightly lower performance than the 7800GTX. It has the advantages of being cheaper and cooler than the 7800GTX. Very good choice of card.
By the way, the 7800 comes in the following PCI-E versions: 7800GT, 7800GTX, 7800GTX-512. I'd go for a 7900 instead though.