At my previous employer, we needed a set of 4 test machines, and we just happened to have an empty rack in the corner of the office, sort of sitting there looking like some unused furniture (this was an open plan, development office).
So, boss thinks.. "Hmm, I can rent 4 compaq mini-desktops for £50 a month, or I can rent 4 compaq racks of the same spec for £150 a month. I'll go with the racks".
Sometime later, the 2U servers arrive and are installed, and powered up... it took about 15 minutes for the number of complaints from everyone to reach the level where we *had* to power them down, they were like aircraft taking off in our quiet office.
The rack had to be relocated to the server room upstairs, where they found the aircon wasn't good enough and so had to be replaced.
And, to really rub it in, we bought 8 of the mini-desktop case PCs for use in the performance lab, and not only were they much cheaper but they stacked on top of each other very nicely, and were silent.
So. You can buy a server case if you want, but you might as well just get a £10 case for it instead (or get a nice Antec Sonata or BQE which is really silent). Incidentally, I'm sure there are options for VMWare so they do not run 'in the background', You also don't need to run 2 instances, but run your server in the VM session and use the host OS as your desktop (I do this). Obviously you will need lots of memory to support 2 VMs (as you're really running 2 OSes on the 1 OS, so you will need 2 OSes worth of memory to run them both effectively)
If you're running 1 user on this machine, then you don't need super-good components, commodity sata drives will be as good as scsi but a lot cheaper and easier to use.