Many people here on these forums suggest that crucial.com/uk is the best place to buy RAM. Certainly I have never had any problems with them, although they are a little more expensive than others.
OEM means different things at different times - eg with software, it means that you have to look to the provider (not the manufacturer) for help. With hardware it usually means that the piece of equipment comes without any of the accesories which might be in a retail box, eg with a CD-RW drive, an OEM might only have the drive itself, and no screws, cables etc which would be in the retail version.
This thread is now locked and can not be replied to.