My current setup is 2x 5770s overlocked and 1xAMD Sempron 130, and I have a 530W 80+ certified PSU. I have space for 1 more GPU on my motherboard. When buying my components I followed a guide that suggested using the 530W PSU for 3 5770s, but I was thinking of getting a 5830 or 5850 instead for the 3rd GPU. How much more power, if any, would I need if I chose one of these cards instead of another 5770?
Do you have a Kill-A-Watt available to see how much headroom you have?
I'd highly suggest a 5850 over a 5830 since the TDP difference between a 5770 and 5850 is 50W. Though you might be fine with a 5830 still... Hard to say.