If possible, tell us your CPU, RAM, and Power Supply Wattage. We also need to know your resolution.
In general, the best card for $100 is the Radeon 6770. After rebate, you can get a Radeon 6790 for that price. For just a little more, you can get a Radeon 6850, which many people consider worth it. Each one improves on the previous by roughly 15%.
But give the rest of your system details as a 6770 may or may not be enough depending on what you have.