as long as it puts out enough watts, even a cheap one can last for a long while before burning out. except for the recently built pc thats in my sig, i've always bought cheap PSUs and never had a problem with them burning out from over use (typically run my PCs 24/7). of course, experiences often differ between people.
when the psu went out in in the HP we had long ago, the reboots started happening occasionally, like once every few hours. the next day, the pc would just shut off as if the power was cut. maybe 2 days later, the pc just wouldn't turn on. trying to turn on the pc would just result in a click noise coming from the psu and then nothing else. total time only took maybe 3 or 4 days. had an ethernet card, soundcard, video card, 2 hard drives, and a cd-rw in the pc at the time. the power supply that was in the pc was only 80 watts.
something that nobody asked yet...
when you say the pc is rebooting, do you mean it just completly shuts off as if the plug was pulled then starts booting?
or is it going threw the reboot procedure as if you hit Start->Shut Down and hit Restart pc?
if its doing the 2nd thing, its probably more of a software issue.
(yea yea, it may sound like a dumb question, but eliminating as much as you can always makes finding problems easier)