Is that processor overclocked? If not, I suggest you overclock it a little, just because even if you buy the GTS250, which is slower than the Radeon 4850, you will be bottlenecked by the processor.
GTS 250 = GeForce 8800GT = GeForce 9800GT = They are IDENTICAL.
9800GT is an OCed, smaller nm version of 8800GT
98000GTX is an OCed, slightly upgraded, and smaller nm version of the 8800GTS
9800GTX+ is an OCed and smaller nm version of the 9800GTX.
The GTS250 is an OCed, less power consuming, and smaller card size version of the 9800GTX+
8800GT/9800GT does NOT = GTS250