to answer your question, yes, you may upgrade this. I called microsoft asking a similar question. here are some key pts to follow:
A) upgrading < clean install: reason for this is that you get better performance with a clean install
B) upgrade vs full version: this was my question, because i have vista home prem and i bought win 7 pro, and i was wondering since i got it for 30 dollars (win741.com) and it was an upgrade. the man at microsoft said the upgrade can be used to upgrade ANY OS. full is for a computer that has no current OS installed on the computer.
C) This doesnt imply to you, but if you upgrade, you cant downgrade upgrade (haha that sounds weird). meaning if you have vista ultimate, you cant use 7 home prem, or likewise ultimate 64 bit with pro 32 bit.
hope this helped!!