The easiest way to get a box loaded with Hyper-V, is to download the "Windows Server 2008 Datacenter, Enterprise and Standard (x64) - DVD" ISO from your MSDN Subscriber Downloads site, build your box, add the Hyper-V role... and go. That's not to say there aren't other options... and 2008 Core certainly is an attractive option. But it obviously depends on your needs, and the the scale of what you're doing. If you just want to build a box with Hyper-V to kick the tires... here is a DO-NOT list to keep in mind...
- Don't install any version of the 32-bit 2008 and expect Hyper-V to work... it only works on 64-bit hardware.
- Don't install "2008 x64 without Hyper-V" and expect to upgrade to Hyper-V... you can't (or if you can, the how is not obvious).
- If you don't want 2008 Core... don't download Hyper-V from http://www.microsoft.com/downloads because it only installs 2008 Core.
- Probably obvious... but don't install 2008 x64 Core and expect to manage Hyper-V using an MMC from the console of the core... you'll need a Vista box with the MMC installed.
And the biggest don't I can think of...
Don't ever start a project without a plan (even a "kick the tires" project)! Because your most valuable resource is time! Ten minutes of reading can save you hours of wasted time!