My system's just a prebuilt Acer Aspire M3920 (12gb RAM, stock motherboard etc) with an added Radeon HD 6670 and Thermaltake 500w power supply. I'm looking to spend up to $300 with the least amount of bottlenecking possible.
You probably want a Radeon 7850 then. Get one with 2 fans, they OC better.
Also, make sure your power supply has a 6 pin connector or 2 spare molex (4 pin ) connectors.