The 8800GTS is essentially an 8800GT with a better stock cooler (with a bit more shader pipelines, but reviews that compare the GT and the GTS clock-for-clock, it reveals that it's not much better). In short, the 8800GTS is not more "future proof" than the 8800GT.
An 8800GT at a good price (around $250 or less) would be great bang for your buck. You could get an EVGA card in hopes to step up, but that's assuming there'll be the fabled Geforce 9 cards out before your three month step-up period expires.