The 8800GT, 9800GT (GTS, GTX, GTX+, etc...), and the GTS 250 all use the G92 GPU. But the GTS 250 is a quite capable card. I believe they added more shader cores and increased the core/memory/shader clocks.
Found this topic here not sure if it is accurate though:
http://answers.yahoo.com/question/index?qid=20090314155750AAPbmsL
If the GTS is better than the 4830 I think I might just got for that one. I spent 102 bucks on the 4830. I am willing to pay a few more bucks give or take if it is better.
You see any you would recommend on the tiger site? Or a brand preference? I would like to try and get one with a HDMI plug if possible so I can run sound through my TV with less wiring.