I'd also go for the 7900GT. I made the same decision back when I got my single 7800GT. Now I'm looking for a 2nd one used... aka hopefully cheap.
Though if you're going for a single card for now, I would also consider the ATI X1900XT. I've always been an nvidia guy, but the performance is sure there. It's about on par with an well-overclocked 7900GT or a stock 7900GTX. For $280ish.