So it looks like you have a PCI-E slot. there's a v1.0 and a v2.0. I have no idea which one this is. SInce you're running a 8300GS, I think it's a v2.0.
For $100, if you want to buy new, I've seen GTS250s around that price. Toms has a best gaming GPU for the money guide, you should check that. I don't know if there are any better cards used at $100- maybe 4870?
The other thing is that you'll probably need a power supply with a 6 pin PCI-E connector. If your PSU already has one and is rated at least 400W, you're probably set. 350W would be cutting it close, and 300W is probably a non-starter.
EDIT: looks like your system comes with a 300W PSU, you probably will need to buy a new one with more power. at least 400W if you're going to buy one.
Okay I just took the thing apart and it's a 330W PSU, I don't think it has that PCI-E connector thingy but the card I'm using now doesn't use one does it? and Helltech the card you reccomended will work fine the way my system is now and will run that game I mentioned?