With 12gb, you will need a 64 bit version.
Windows-7 home premium will do the job.
If you are a student with a .edu e-mail, you can get an academic license for ultimate cheap.
You can get the retail version which Is more expensive, but gives you portability to a new pc later.
The oem version limits you to an install on the original mobo only, and does not give you the MS support of retail. It is cheapest.
Another option is the upgrade version which is considered retail, and costs about the same as oem. You should have a previous OS, but that requirement is flexible.
An interesting last option is the home premium upgrade 3 pack for $125. Your cost per os is the cheapest, and a great deal if you have another pc to upgrade.