For $100 your going to be looking at something like >
GTX 460 Video Card - 1024MB, GDDR5, PCI-Express
Radeon HD 6770 Video Card - 1024MB, GDDR5, PCI-Express
Each around $100 at Tiger Direct USA
Do check power requirements on new Graphics cards and your PSU wattage and cables, also consider your case size.
You may find this handy > http://www.videocardbenchmark.net/high_end_gpus.html