Hyper-V is in competition with VMWare. VMWare 1.5 (whatever) does not run on 64 bit OS and Hyper-V only runs on the 64 bit version of 2008.
In the production environment at work, we currently use free VMWare server to run some client test / demo systems (we develop cataloguing software).
I am recommending we use upgread from Enterprise 2003 to the Enterprise version of Server 2008. We currenly run the VM demos on a two disk striped disk (not mirror, mirrow slows down the OS something shocking) for disk access speed. I plan to upgrade the RAM from 8 gig to maybe 16 gig (Enterprise 32 bit supports it, but not Standard edition) which will increase speed and still allow us to use VMWare (not supported on 64 bit).
The killer feature for me on Server 08 is the Bare Metal restore (restore to new hardware). I can backup to network share, then backup the files to tape (off site) and if a hard drive files I can just quickly reinstall the OS to the same server or a different server. Simply Awesome.
I intend to document a step by step process for the bare metal restore soon.
Hope this helps a bit.