NEVER, I REPEAT NEVER! go cheap on your PSU ... especially for what we use them for! You want to get a very nice PSU because often times in the PSU world "You get what you pay for" applies very well.
I would recommend reading reviews of PSU's where they actually put load on the PSU, measure the voltage regulation of each 12v 5v and 3.3v. Also they measure ripple.
One of my favorite manufacturers (Seasonic) OEM's for many different PSU manufacturers with higher priced PSU's.
Many of the Corsair line is Seasonic I believe.
Seasonic makes some of the most stable / quality PSU's in the 400-800watt range.
There are a couple of other really good OEM's out there, but my experience is mostly w/ Seasonic.
Superflower is also making a comeback. Releasing extremely high quality PSU's under several different brands.
Either way, stay away from your 40$ 600watt PSU's they are utter garbage and most likely will destroy your PC. For a nice 600watt PSU plan on spending a good 70-90$+
2 sites I love for quality, unbiased reviews:http://www.jonnyguru.com/modules.php?name=NDReviews&op=Review_Cat&recatnum=13http://www.hardwaresecrets.com/page/power
+ Seasonic. Great PSU. Excellent customer service!