You can only run the 64bit version if you have a 64bit processor, and even then I'd be wary. I've got an FX-55 which is 64bit and I'm not going to install 64bit Vista - I'm too concerned about compatibility with a largely 32bit world.
OEM stands for original equipment manufacturer, it's the version installed by the computer makers such as Dell, Mesh etc. You can buy an OEM version if you're building a PC and it will come without the pretty packaging and any additional bits and pieces. You can't (or shouldn't be able to) buy an OEM copy unless you're buying hardware to build the PC with.