The NRP-VC502 was 80+ Bronze, was made by HEC (a very hit/miss OEM), and was not very well reviewed by one of the top PSU review sites: http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=158
The VC503 is made by Solytech, isn't even 80+ certified, and I can't fin a review from any site...much less a reputable review site. I personally wouldn't stick this in any computer that has as much money as in it as the rest of your parts. What is your budget for the PSU? Are you buying in the US? If not, then can you give a site for us to look at and give recommendations?