Hold off on buying any of those, it could be a waste of cash. When you purchase Vista, you're automatically entitled to both 32 and 64 bit versions. Simply go to
this website, throw in your key, pay the $10 shipping fee and they'll send you a 64-bit installation disc. Your current key will work on the 64 bit version.
To answer your question though, there is no real difference between retail and OEM copies of operating systems. The difference lies in licensing, like you mentioned. OEM installs will require you to buy another license if you make a "major hardware change" which generally means a new motherboard. This can sometimes be worked-around if you sweet-talk a Microsoft rep, but for the most part it holds true. Retail copy licenses can be moved from computer to computer as long as only one is in use at a time.
Both OSs you linked are OEM builds. But anyway, don't buy either until you try to get hooked up with the alternate media first.
(Also 8 gigs of RAM is a waste unless you do a heavy amount of programming and graphic rendering. To play games, 4 is plenty, and 64 bit Vista will let you address all of it)