Best version for gaming
Alright, so I know that Windows 8 is coming out in less than a month but which version of Windows should I get for a system that I want to use for gaming knowing that my current version of Windows is XP Pro and I intend to get a system that has 8 GB of memory. Should I buy an OEM version of an OS or a regular edition, 32 or 64 bit?
If you have a gaming system you want to use, then Windows 7 or XP is probably best. Most games will not 64-bit yet and will not be able to address more than 4GB RAM, but since you have 8GB RAM in your system, the 64-bit version is recomended. WinXP is more compatible with older games and Win7 with newer games. Win7 can also take better advantage of your hardware than XP so overall, I'd recomened Win7 - Hm Prem, Pro or Ult editions
Windows 7 is your best choice.
Aside from anything else, Windows 8 has compatibility issues and so far poorer gaming benchmarks.
XP goes EOL in April 2014 and doesn't support the latest DirectX versions.
The comments above regarding games not able to address greater than 4GB of RAM are correct and incorrect. Games do not address RAM at all, an operating system does.
For your OS to address 8GB of RAM you will need to use the 64bit version.
OEM licenses are tied to the motherboard. If you plan to change your motherboard and take Windows with you to your new machine then you will want a Retail version that allows this.
When you purchase Windows 7 the key will allow you to install both the 32 and 64 bit versions.
EDIT: Also regarding the specific version you generally only need 'Home Premium'. Pro is for business (joining domains etc) ultimate has some tools for encryption that are as useful to a gamer as a tin opener is to my cat.