HD 7750 doesn't require any external power connectors. This card is often recommended for 300 watt PS.
Using Extreme Power Supply Calculator and your system info, you could go up to HD 7770, HD 6850/6870 if you don't add anything else to your PC. Personally, I would go with the power efficient HD 7770 which performs similarly to HD 6850. No overclocking is assumed.