Hello,
I recently built a extreme low buget computer for a friend and although the parts were on the lower end of entry level the performance I gpt for $$ was darn right amazing.
What I got was a P4 631 processor, 1 Gb ddr2 some G31 asrock. generic mobo and a Palit 8500GT.
Everything worked and so far my friend can paly any game he wants to at decent settings (fifa, cs source, COD2, BF, NFS series).
So to answer your question, a 8500Gt should do just fine with your rig and for cheap too. Just take a look at these deals:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814143104
Very good price after rebate and should deliver decent performance. This is the one I would recomend for your needs.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127295
This comes closer to the one I bought for my friend. Still a good price after rebate and has about 30% more performance than the BFG.
At any rate these cards will give you dx10 compatibility (but don't expect any dx10 gaming performance) and should run dx9 games pretty well, and for around 50$
Moving up into the 50-100$ segment you could also consider some older gen video cards such as this one
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150185
But in reality this 70$ card fares just a little better than the overclocked 50$ MSI 8500GT.
Or perhaps the newer 8600GT which will run ANY game out there today (you'll play these games on medium though)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150229
This is basically an overclocked 8600GT with a good bundle and should serve you well.
All in all I'd say the 8500GT is the best card for your needs but if you need help determining the bang for the buck you could check the following resources:
http://www.gpureview.com/show_cards.php?card1=385&card2=514#
Here you can check the raw power of different GPU's and even factor in different clock speeds.
http://www23.tomshardware.com/graphics_2007.html?modelx=33&model1=1066&model2=716&chart=318
A series of benchmarks to help you gauge the real life perfomance of differend GPU's.
Hope this helps.