I just want you to do, I am a professional in the feild regarding the following GPU's as of a few days ago. I did 2 months of research for the 8800GT/9800GT/9800GTX+/GTS250. (thanks to all the people on the forum.)
You know what I found out? I found out that the 9800GT was a re-badged 8800GT (yes, literally, physically just a label change.) and the GTS250 was a re-badged 9800GTX+
Now there's a notable and noticeable difference between the 8800GT and the GTS250. The fact is by default the 8800GT is @ 600/1800 and by default the GTS250 is @ 750/2200. So there is a difference, but if the GTS250 costs $30-$40 bucks more then a 8800GT, I wouldn't recommend it because you can buy a used 8800GT for $50 bucks and OC it yourself to that speed. BUT, I must give it to you, most GTS250's come with good heat sinks.
It all depends where you live, and how far your willing to go. Overall, I would recommend the GTS250, but if on a budget, go with a used 8800GT + good aftermarket cooler.
Here is the comparison for the 8800GT and GTS250, as you can see, there is a notable difference in Bandwidth, fill, texture and flop rates. So yes, if you have no budget then I would reccommend the GTS250.
http://www.gpureview.com/show_cards.php?card1=544&card2=606
Hope this helps.