oem or generic PSUs (or even bad brands..) are very cheap quality, and can destroy your system. they tend to be unstable. it says 500watts but with oem's its usually exagerated. you can have high-quality power supplies that don't give enough power, but they won't harm your system. for generic power supplies, not only can they be insufficient, but may also damage your expensive components.
keep away from OEM or no-name brand power supplies.
get from companies like antec, enermax... and umm there are lots more just can't think of em