The problem with GTS250 is the temperature and the watts ( corect me if im wrong) however is a good video card is lik to have a HD5750
The 9600GT is not bad, low price and good performance, though is quite old. ( now that i check, this one is slower than your GFX!)
ATI Radeon HD 4670: a friend of mine got this one, is good prformance, it doesnt heat, and low consumption and is for run games on low and medium quality. Like the 9600GT ( this one is slower than your GFX!)
ATI Radeon HD 4850 is a good video card, med/high performance if you like this one try to get more money and go for a 4870 that is better than a 5750.
ATI Radeon HD 5670 is quite good, a new gfx and supports DX11 but is slower than the 4850.
remeber that these video cards ( except from 5670) dont support DX11...
and the GTS250 supports Physx
In my opinion I will be between these:
GTS 250 ( if you have a good power suply and low temperature computer case)
HD4870 ( will be better if you can get a bit more of money)