retail- you get the manufacturer's warranty, not just the retailers. you get the actual product box, sealed, with any software/accessories the manufacture wishes to bundle with it. retail products also cost more.
oem- cheaper price. you get only the retailer's warranty. no product box. usually ships with the bare necessaties: drivers, manual, etc.
i don't know of the actual hardware being altered between retail and oem. but i have heard that the radeon 8500 retail is oc'ed? or at least has a higher clock than the oem version.