Which version of windows should i get????

The System Builders OEM edition is licensed to be installed on one computer and one computer only. So if you change computers, you need to buy a new copy of Windows per the license.

Retail can also only be installed on one computer at a time, but if you buy a new computer, you can delete the Windows installation on your old computer and reinstall it on your new computer. You do not need to buy a new copy provided you delete the old copy (so you can't sell/gift/donate your old computer with Windows still on it).

As to what version to buy, I remain a fan of Windows XP 32-bit edition. I do have the 32-bit and 64-bit versions of Windows Vista, but XP always served me well when it was my only OS and now that almost everything I do I can do in OS X, my needs in Windows are very basic so XP allows me to use less memory and HDD space vs. Vista.

For performance reasons alone, don't touch Vista. Go with XP and you'll be able to do anything you need to do. I don't see Microsoft dropping support for XP for a while given new systems are technically still being sold with it pre-installed.