You'd need a power supply upgrade. I don't know of any possible configuration wherein you could fit the whole system plus a decent discrete card for 250W.
After the PSU upgrade, the 9600GT should be better than the GT220. Or you could also consider the 4670.
but here's what i found in Tomshardware :
GeForce 9600 GT (1024 MB)
31.3(full 2D Load Wattage)
68.4(full 3D Load Wattage)
5.7(Current in Amps)
don't u think my 250W Hipro should be enough then?