if you're using an old generic 350w psu the your monitor is plugged into that, then you might have problem with that.
if you're using a tru-rated (the one that doesnt have power slot for monitor) 500w generic psu (not well-known brand), it will do the job also.
if you're using a tru-rated "branded" (corsair, coolermaster, ttake, fsp, antec, & the like) 400w psu, then you're good with that.