Which is the better hardware? OEM or one made by a company? I'm thinking of buying a Radeon 9800 pro, but are not sure what the difference between OWM and normal is.

OEM versions are those straight from the company and cheaper, they are something like "pure" since the company did not do any tweaks or modifications to it. However OEM versions are not as good as those from manufactuers.

Cards from manufactuers (Eg. MSI, Ledtek, Gainward, Gigabyte..etc) usually tweaks the video card to let it have more features that the OEM doesnt have, and of course its more expensive.