for heavy gaming I would recommend you wait 'till you gather some more money. If you had all the right specs and the only thing you needed was a graphics card, then you can do quite a bit around $100. But most likely from those specs you are going to need a new PSU and CPU. witch means you would go far over budget.
this is a good card for the money http://www.newegg.com/Product/Product.aspx?Item=N82E16814102908
that CPU will almost certainly bottleneck it. Meaning That the slower CPU will slow this card down. I would suggest a quad core rather than a dual. AMD makes some good CPU's for good money. You don't REALLY need a fantastic CPU for gaming. It does certainly help, but usually the graphics card is more important.
If you really can't spend the extra money (I really can understand.) then the radeon 5770 isn't a bad card either.
Also, does your motherboard support PCI express 2.0 or 2.1? That is rather important.