Windows 7. Which version should I get?

Yes, it’s confusing even for me. I’ve googled the hell out of it and can’t figure it out.

I have Windows XP (pre-SP1, won’t even boot on my computer) and a valid serial key.

I have Windows 7 RC installed (which will start auto-shutting down every two hours starting March 1).

Can I purchase an upgrade or do I need the full version?

The RC is essentially Ultimate edition as I understand it. Is this correct?

Which edition should I get? I don’t really need BitLocker and I really just run apps on it. Can I get the most basic version or would you recommend a higher level edition for a power user like me? (I’m a power user, right?)

In case it’s handy, here’s mainly what I use the PC for:
* Modern Warfare 2 and Counter-Strike: Source (yeah, yeah—it’s not too often)
* VMs (this is the primary usage)
* X-Plane and Python development
* Windows browsers for testing and web development
* Any Windows-only programs I have to run since I hate doing VMs on a laptop

Also, is there any way to just purchase the license and not have to reinstall?