The 7800 series really excel when in SLI. In fact, everybody I know that has a 7800 series card does SLI them. I would go with the 7800GTX if I was going to get one....the GT is crippled, although you can unlock the pipes from what I hear. I learned from experience its best not to get a card thats crippled for the reason of making it cheaper.
I went with the 6800GT with 256mb.. For the following reason, cost, performance and it will suit me needs. Also the card was the evga, if it doesn't work out I can always upgrade later on.
