OEM products are usually different only in that they don't have any of the fancy packaging. For example, if you buy an OEM P3 FC from a local shop, you get nothing but the CPU embedded in foam, and a static bag. Same thing for HD's, video cards, etc. Also, OEM parts usually don't have all the frills that retail parts have. OEM video cards, for example, rarely have TV out.
For the most part, OEM is the way to go for a DIY system build. Recently, though, there was a big flap about ATI shipping OEM boards at a lower clock speed than their retail counterparts.
Viper GTS