the psu is a very important part of a pc and it is best to get a new quality unit. cheap psu's do not give their quoted output. In this months custom pc mag they tested a range of psu's and the cheap generic 400w could not manage 372w, plus it drew 385w from the mains making it very inefficient.
i replaced my 300 watt psu with a cheap 450 watt one and got rid of it after a day, it ran like a stalling jet engine, never again! best advice given to me (from here) was dont cut corners on your psu!