Technet Vs Retail Keys

I have several computers still running the RC version of win7 and I am trying to pick the most cost effective option for installing legit version.

I had first thought to get the 3key Family Pack, but have obviously far missed my window of opportunity on that option.

I rebuild and test new computer equipment frequently, so I don't want to use a OEM version that is limited to one activation (unless that's changed?)

I've heard mixed info that the retail version is good for 10 activations, is that for 1 PC or multiple PCs running at the same time?

I tend to run 3-4 towers at once at minimum in my home so I need to know if I need separate keys for each PC I am running. Are the activations good, if I make hardware changes to my PC? Swapping motherboards is always a concern so I don't want to be shelling out for a new copy every time I try out a new board.

I have heard that a technet subscription might be an option for me, but I am concerned my use of the 'Evaluation Copy' software isn't really part of the license agreement. I wouldn't be evaluating the windows software, it's normally my hardware I am evaluating.

Can someone give me more infomation on how the Technet accounts work?

I know if I need different keys for each of my PCs then I would probably need 6-7 keys, I don't mind paying $300 a year for a technet subsribtion if my use of it won't be a problem. But I don't want to shell out $300 only to find out should have bought the retail version instead. And then again if the retail version is good for 10 activations, and I can use it on multiple PCs at once, that would also work for me.