Ive found that a sure sign your power supply is inadequate, is put everything under load, your computer will either A) Reboot/Blue Screen or B) You will hear the fan(s) slowing down (as I did in mine)


There are some sites that do a rough calculation for you, if you enter in what you have.

My suggestion is buy a decent one though, dont go for the cheapest one, every cheap one Ive had hasnt lasted very long.