Windows 7 Window 7 Purchase Question.....

A good friend of mine gave me a copy of win 7 Ult. 32 bit to try out on my laptop (its not fast enough to run 64bit). Anyway, I installed it on a new hard drive that i had and I think it great, I've used the extension for running it for 120 days. My issue, since I have the win 7 Ult. already installed on a HDD should I just purchase a product key for it ($100 cheapest I found) or purchase win 7 pro disk (oem or full version $124 found online). I guess my main issue is since I already have it installed, I would like to just activate it with a new product key and keep on truckin'. On the other hand, a clean in stall, a full version or oem disk...major debate, price is about the same......anyone have an opinion on my issue.

Thanks