Is it a standard or low profile machine?
You can get a good PNY Geforce 430 in either profile for well under 100 bucks. Its not a screamer card, but a solid one for most games. Even though its not "hardcore" - it doesn't require a PS plug, and its fast enough that it won't be the bottleneck in your current rig. You would need a MB/CPU/Ram upgrade before you throttle the card, so for the price its worth it depending on your long range plans.
$63 bucks here:
http://www.amazon.com/PNY-GeForce-PC...9276890&sr=8-1
I have used the card and recommend it highly for the use your looking at. I actually put this in my Low Profile machine at work (its my "test" machine that has a few games on it - for BENCHMARKING purposes only, I assure you!) and it handles games like World in Conflict, Dragon Age /DA2, etc without issue. Not on highest settings or max rez, but still looks really good.