It appears that you lose the ability to run compatability mode and join a domain, remote desktop, by running the home premium version... nothing notable really.

http://promotions.newegg.com/Microso.../Products.html

The best bet for 5 boxers is likely to be two home premium family packs...

http://windowsteamblog.com/blogs/win...k-pricing.aspx

I'll definitely be buying a family pack myself to upgrade my wifes PC and our DVR, but since I'm throwing a launch party myself I got a freebie Ultimate license.