can I use the windows 7 UPGRADE version for this?

No, you need a retail copy, you can use an OEM copy (unless tied to a manufacturer) but you can't use an ugrade and stay within the terms of the license.

If it is a full installer and you have a prior legal copy, yes, but in order to activate you will actually need to install it, then reinstal again over itself is all.

I had Vista Ultimate with 'free' upgrade DVD for when Windows 7 came out (ws in the months before 7) looked like a good deal... but I was using Windows 7 RC (beta) and installing Vista was a real pain.

As the hatter said, you CAN use the Upgrade version of Windows 7. Here's more details:

Install Windows 7 normally, using Boot Camp and your Win 7 Upgrade disk. Use the "Custom (Advanced)" option.

When it asks you to enter your product key / license, don't enter anything -- just hit Next. This will allow you to continue installing Windows with a 30-day trial.

After you've installed Windows 7, stay in Windows (don't boot to OSX) and put your Win 7 Upgrade disk back in the drive, and run the Setup file that's on the disk.

This will guide you through the upgrade process. This time, instead of Custom, use the "Upgrade" option.

When it asks you, go ahead and type in your product key, and choose to Activate.

However, please note that if you do this, and you DON'T already own a previous version of Windows (and stop using that previous version immediately), you'll be violating the software license agreement. These are the requirements to use the Upgrade version. If you don't meet these requirements, I strongly recommend you install with a full version of Windows 7.