well first of all a basic rule of thumb is that any modern computer should have a PSU of at the lowest 350W, but before replacing your psu i would run your computer and check your voltage rails with either bios monitoring, Asus Probe, or a DMM, if you have plenty of power under a load (my 550W psu has a 12V rail of 12.12V, 5V rail of 5.1V and 3.3V rail of 3.32V), then the psu should be fine, 12V rail should be 11.6 at the lowest ever, if it drops below that you definately need to replace the psu..
other possible situations:
try using only one stick of ram, you could be having a faulty ram situation
if you have access to a space video card, try using it for a while
*remember troubleshooting is all about testing single variables!
Asus A8N-SLI Deluxe, AMD64 Winchester 3200+, 2GB Kingston DDR400, 2 6600GTs in SLI, 2 WD 80GB SATA HDDs in RAID0, Aspire 550W PSU, Microsoft Windows XP Professional x64 Edition, Thermaltake XaserV V7000A Case