WHS 2011 is based upon 2008 R2 and indeed reports itself as 2008 R2 for compatability.
I have mine fitted with 8GB RAM, 4 3TB drives plus the 250Gb drive that it is supplied with for the OS and have recently fitted it with a GT430 GFX, Im running Windows Home Server 2011, with Flexraid setup as a drive pool and Snapshot Raid config, the Flexraid setup is far superior to the Raid 5 you are contemplating.
It does require a custom BIOS to be flashed to enable full SATA on the ODD port (where most people put the OS drive) but no big deal to sort it out.
Its a great little server for the money IMO especially if you can get the £100 cashback