Gts 250..

Decent enough for resolutions upto 1680x1050. To do justice to your system though, consider upgrading to hd5850/gtx 470, about two times better than your current gpu.

okay, your system is good. but:
1. what about monitor? what size?
2. How many sticks of RAM do you have?

Depends on what you want to do with it, but looking at the rest of your system I think the GTS250 will be a touch disappointing by comparison.

i have 4gb ddr3 @1333Mhz and samsung syncmaster t220hd

21.5'

and please consider that i want something not ultra expensive

Yes, GTS 250 is a good card for your system, at least for now...
If you want to see performance increase then HD5770 is a good upgrade.