|You won't be able to overclock your system because it uses a custom/locked eMachines BIOS.|
Take a look at the Tech Report's Econobox. It's a little over $550, so if staying under is a must, you could go for a Radeon HD 6750 instead of the suggested 7770. Or you could reuse some of the components in your current system, such as the DVD drive and hard drive.
HTPC | Pentium M @ 2.82GHz, 2MB L2) | 4GB | 1.0TB | Radeon HD5750 | Blu-Ray
Win 7 Pro | Modified PowerMac G4 QuickSilver case