Homebuilt server for Hyper-V or ESXi

The requirements to run ESXi and Hyper-V are rather different, i.e., unlike Hyper-V, ESXi hardware support is somewhat limited (in particular NICs and RAID controllers). Windows Server 2008 Hyper-V will run on lots of motherboards and the on-board RAID is supported. Your main concern is stability (crashing VMs should be avoided).

I use motherboards that are good for overclocking (I don't overclock them to make sure they are stable) and very good memory. I tested Windows Server 2008 using a Gigabyte GA-EP45-UD3P (my current VM server) and an Asus P5Q Deluxe. I also built a server based on a Biostar TPower I45 for Windows Server 2008 R2 Hyper-V. All systems use 8 GB of DDR2-800 G.Skill memory. If I had to build a new test server, I would probably use the GA-EP45-UD3P again with a Q8400 or faster CPU, but VMs in a test environment usually don't require a lot of CPU power. Your hardware requirements might be different, but I run only a few VMs at a time and all they do is run Oracle and Tomcat.

If you can afford it, buy the fastest hard disks that you can afford because that's where the bottleneck often is.