7800GTX - X1950XTX = 20-40% increase in performance for $350.
7800GTX - 8800GTS = 40-80% increase in performance for $400.
Plus the 8800GTS gives you DX10 compatability. Why would you trow you money away on the ATI card? IMO, I would wait, the 7800GTX gives decent performance. What specifically are you playing that makes you want to upgrade?
Currently I'm playing a lot of Call of Duty 2 - I get very decent frames but just when going through smoke I'd like it to be a bit more stable.
I was just interested to see what would be more cost effective, to upgrade now and still receive a reasonable price for my card from the shop and then upgrade again later from the ati card. Rather than wait and get a bad price for my card and have to pay out more for the dx10 cards.
You've given me an idea though, i hadn't really considered the gts :?