How big a PSU do I need?

i5 650 has 73 watts TDP. ATI HD 5770 draws up to 108W. The rest of the system won't consume 100W. I bet that set up will never hit 350W on wall socket. A 400W power supply with about 25A on +12V rail would be enough but a bigger 500-550W unit gives you elbow room for future upgrades.