I started on an GS500. *Perfect* bike for twisties. After only 3 months of riding I went up to the North Ga mountains with my roommate (95 VFR-750, been riding for 15+ years) and a friend of mine (97 GSXR-600, been riding for 6 years) and I was leaving them in the dust on all the tight turns and chicanes up there. The GS-500 is so easy to flick back and forth (mostly due to that 130 or 140 rear tire) that it was easy for me to lean the bike over and point it where it had to go. No way could I have done that with only 3-months under my belt on a more advanced bike.

Ride the GS till March-April'ish (assuming you get at least 2000-3000 miles on it) and then upgrade. In the meantime, try to sit on or test ride every model of bike that you can. I'm willing to bet that you'll have a different opinion of what you want to upgrade to.