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are you looking for upgrades for games?
or for something else thats memory intensive... (ie photoshop, auto CAD etc...)
or maybe just multi task a lot :P
for games, I'd think your video card is more important
I don't see how a bigger power supply unit (PSU) would improve performance unless you are
1. adding new hardware
2. upgrading hardware
3. upgrading to a 80PLUS certified PSU to save on electricity bill
also... some manufactures have proprietary PSU