ya one card is good enough.
you were saying SLI is better than crossfire, which is true, but irrelevant to you since you're only going to be using 1 card.
if you compared the 7900gt to x1900xt, the x1900xt is better.
if you eventually some time in the future want to double up your card and run in SLI.. then the 7900gt makes sense. if not, and you have the money, the x1900xt makes sense.