anyone with experience with open license?

I had a question I am hoping someone who's installed open license can
answer.

The open license is an upgrade and you have to buy the media seperately. I
understand that. However when buying media there are upgrade and full
versions.

It's been a while since I've done this but on the "retail" license I believe
I've been able to install either upgrade or full version using an upgrade
disk or vise versa.

My question is 2 fold
Does it matter what media I buy? Does it have to be upgrade since
technically the license is an upgrade or do I buy full?

The second question is when installing the open license I know that you get
one license you can use to image all your pcs with so you don't have install
each individually or change the image with a new key for each clone. But my
question is during install is it similar to a retail installation where it
verifies media or checks for existing install or is it different.

The other reason I ask is because I'm allowed to use windows XP with the
open license and then later on I can upgrade to Vista. Since I already have
XP discs both retail and OEM maybe I don't have to buy the XP media now, and
only the vista media before the 2 year window expires?

Thanks,
Robbie