all the above is true...but add this to it also.
With a retail version of xp (home/pro) you are supplied with full Microsoft Support features without charge for a year and also as you grow to another computer the retail version is allowed to travel with you to a different computer.
The OEM version has absolutely no support from Micrsoft dealing with any system issues. You will get updates but no personal support. Also the OEM version can only be installed on one computer (no exceptions) it cannot be removed and reinstalled on a different computer.