"Some PSUs have huge 3.3V and 5V ratings to make themselves look good. One should not be tricked by this marketing trick, as the rating is assuming individual drainage. What it means is that only 3.3V or 5V is being drained. This is hardly the case as your computer drains both rails. The buyer needs to look at the combined rating for the 3.3V and 5V rail. It is usually 150Watt for a good 250Watt PSU and 170~180Watt for a 300 Watt one. This rating is the most important bit for the stability of your high power CPU or video card, as they both draw power on these two rails. If you can not find the combined 3.3V and 5V rating for the PSU, do not buy the unit as any reputable manufacturer should have this."
FULL STORY @ OVERCLOCKERS (http://www.overclockers.co.nz/ocnz/misc/st300blvpsu/f1.shtml)