The GTS 450 would be an excellent choice. I would not go any higher than that without upgrading your PSU. Its not just a question of total wattage, but how much power is on the 12v leg, the age of the PSU, and the multiple advantages of not pushing a PSU near its limits.
Hear are some good THG reviews on the GTS 450 in case you have not read them:
In depth look at GTS 450http://www.tomshardware.com/reviews/geforce-gts-450-gf106-radeon-hd-5750,2734.html
Roundup comparing 6 cardshttp://www.tomshardware.com/reviews/geforce-gts-450-overclocked-roundup,2762.html
In addition, you might want to check out http://www.upgradevideocards.com/
for a guide to upgrading graphics cards.