So how much RAM do you need?
When you are using the full GUI of 2008 r2 as the parent, reserve 1 gig for the needs of the OS. Then carve out the ram for the rest. SBS, Exchange and SQL will not do dynamic memory. I've got a small biz mentality that isn't so sold on "over committing" memory anyway.. I think it's asking for trouble.
SBS 2011 standard - I'd say 16 gigs or more is where you want to have your mind at. SBS 2011 essentials however, you can set that RAM much less.
And remember whatever the inside child is assigned as ram, that's what the outside partner will have in use.

And how much ram should you have in the parent? Remember that win2k8 r2 standard has a max of 32gig. If you use the free non gui HyperV server you can bump up the ram.
Make sure you've installed all bios updates on the parent. Dig into the network card and especially if it's a broadcom, get those latest nic drivers.
http://social.technet.microsoft.com/wiki/contents/articles/365.hyper-v-gotchas.aspx
Because SBS doesn't support nic teaming I tend not to do it. Sometimes keeping it simple means you keep it simple.
So now that you've built the parent, have it in your mind that you got so much ram for each child, you've got a network connection set up on two nics, one is the main nic I'll call the "admin" nic. This is the one you'll RDP into and use for maintenance. Now set up at least another one that will be the nic that you bind the children to.
This took a bit of mind wrapping... as you end up with something I'm going to call the outside nic and an inside nic.
First off on the parent all you install is the HyperV role. That's it. Nothing else. And in fact on the free HyperV server, that's all you are allowed to do. Same with the 1+1 role you get when you get a copy of Win2k8 r2 server and are allowed to lay down a parent HyperV. You start the server, go to Server manager and add the HyperV role.
Now you launch the HyperV manager. On the right hand side you click on Virtual Network manager and you build a network connector. I normally call this External so I know that this connects to the outside.

(this is my test HyperV box at the office). The box that is checked that says "allow management operating system to share this network adapter" is checked that way due to the HyperV wizard - http://blogs.msdn.com/b/taylorb/archive/2009/01/12/hyper-v-v2-guest-only-external-networks-add-roles-wizard-changes.aspx
When it does that ..and this is the part you have to get your head around... that one physical nic turns into two nics and the bindings on each indicates which one has the bindings inside the virtual machine.



See how one has the virtual bindings and one does not?
You need to set up a virtual nic and bind it to a live nic in order to have external internet connectivity

For every physical nic you can bind connections to it. In a small firm ..and especially in a test server you can actually have multiple machines share the same virtual nic. In production you probably want to plan your nic bindings a bit more carefully and not so willy nilly.
Up next...we start to install the SBS Essentials.