I've seen adverts for Hyper-V based virtualised servers, with as little as 512MB RAM. This seems quite small to me, but I'm wondering if anyone has experience of running a virtual server with this amount? Can windows really run efficiently with these specs?
No, but it will run acceptably with only a handful of services, and under low load.
The biggest performance bottleneck will be the operating system paging between disk and such a limited amount of RAM. If the virtual hard drive is on a very fast disk array then performance may be more acceptable.
I've run 2008R2 Virtual servers with 512MB RAM. Admittedly they weren't having a massive load put on them but they ran fine nevertheless.
This was also in VirtualBox though, not in Hyper-V.
The minimum reccomended RAM for 2008R2 is 512MB, depending on the server edition it will support up to 2TB.