It depends on, for what you want to use it
If you only work with CG then a GTS250 is enough
but if you play games etc. i would buy the GTX!
As adozv already said, the higher the price, the more performance you'll have
But there is another reason why i would also buy the GTX, cuz the GTS250 has a old chip, it is the same as the 9800series G92b
NV just changed the name (marketing).
I look up the actuall scores of both GPU's: Game: Crysis Warhead
GTS250 reaches 9 FPS
GTX260 ......."........17 FPS
Only, the GTX has a higher power usage then the GTS (about 50Watts difference) and the temps (GTX ca 70°C) are higher then the GTS(ca. 50°C (both, idle).
Now you can decide
CPU Q9450 @ 3,6Ghz
Memory 4x2GB OCZ Reaper HPC
HDD SSD Vertex2 120GB - Samsung Spinpoint F3 1TB
Graphics Card Gainward 88GT GS 1GB OCed
Monitor Setup 24" Samsung SyncMaster BX2431
OS Win 7 x64 Ultimate