Which Windows 7 Do I Need?

Hello there!
I want to install Windows 7 on my iMac via Bootcamp (Not Parallels, Crossover, etc) but after doing some research I'm really confused about what copy of Windows I should buy. Like, can I install an OEM copy of Windows 7 on my machine? What even is an OEM copy?! And what's the difference between a 32-bit version and 64-bit version of the OS? I think it has something to do with how the CPU handles RAM but I'm still none the wiser as to which one I should get for my mac!
I know I sound clueless but I've only been on the mac for a couple of months now and I'm still pretty tied to the Windows OS so being able to alternate between OSX and Windows is ideal for me right now. I will post my system specs for you, I only got the computer like February this year:

Processor Name: Intel Core i5
Processor Speed: 2.7 GHz
Number of Processors: 1
Total Number of Cores: 4
Memory: 4 GB 1333 MHz DDR3
Graphics: AMD Radeon HD 6770M 512 MB
Software OS X 10.8

Any help is greatly appreciated!
Thanks

You can use an OEM version however it is then tied to that one machine which can be a problem if you purchase another Mac and want Windows 7 on it. Install the 64bit version as it will read all the memory you have installed. A 32bit install is tied to about 2GB. Games that are 32bit will run under the 64bit install.

Also consider upgrading the memory. Same model as your 12GB made a huge difference.

Thank you harryb2448!
I will have to start saving before I think about upgrading my RAM but I will bear it in mind!

Also, what does 'DSP OEI (PC)' mean written in the title for windows 7 on Amazon?

Ram is pretty cheap if you don't buy it from Apple