Try the HD 5570.
It's probably the best option for this machine without switching power supplies. And since this machine has an E5500, I don't think you'll run into any power issues. The e5500 is pretty efficient and low power.
For gaming it won't be that bad, either. It should handle any console port fine enough. Its performance will be like a Radeon HD 4670 and HD 3870/3850.
Try to get the DDR3 version; avoid the DDR2 version.
If possible make the HD 4550 your last resort. Other good options if you can't find an HD 5570 are: Radeon HD 4650, Radeon HD 4670, Geforce GT 220.
All of these should be fine with the existing power supply.