Upgrade: XP----->Windows 7

I think I have this right but let me know if not. I have a new MB and CPU but the same HDD that runs XP. I can't boot XP because it's OEM version and since the new hardware it won't let me.

My question is this..can I buy the upgrade to W7 instead of the retail version and install it? I've already backed up my info and ready to wipe HDD clean. I just want to make sure that I'm not wasting time and $$$ if it's not possible to do a clean install with the upgrade version. I've read conflicting views and it's not very clear on what you can and can't do with the upgrade.

Also with the upgrade version, do you get tech support from Microsoft?