Whoa.......yeah, I coulda told you that the PSU wouldn't have been anywhere close to acceptable if it originated in an HP machine.
You don't necessarily need to spend $200 on a PSU man. If you really want to, then the answer is yes, that's a really good PSU and it will last you for a long time.
As far as it fitting, well.....all PSU's are supposed to be the same size, however, since yours came in the HP I'm guessing the PSU was a proprietary part and will only fit in that HP type case assuming that's what you're using.