If you need to have direct interaction with the desktop on the VM, you probably should not go ESXi. You could certainly go this route and then interact via the vSphere console, but it will be sluggish. You could also remote into the VM, of course, but this is an incredibly overcomplicated solution for what you need. There are also hardware concerns with ESXi. You can spend a lot of time trying to get ESXi to install on non-certified hardware, and it seems time is something of which you do not have an abundance. If you have something on hand that's on VMware's HCL (
http://www.vmware.com/resources/guides.html), you don't need GUI performance (not sure if there is any animation on your VM), and you want to do it just for the sake of doing it, go for it. You'll get the best performance and best use of resources, but I personally think this route is way overkill for what you need. Also, backing up the VM will be not be nearly as easy as what you're used to now. You'll end up having to use hacks or third party software for ESXi (again, that means more time). You really don't want to use ESX/ESXi unless you are dealing with multiple VM's, are really wanting to squeeze the absolute most out of a given piece of hardware, and want access to a ton of features that are only relevant in enterprise (ESX only, not the free version).
My personal preference is VirtualBox for what you're doing, but since you're already up and running with Virtual PC, why not just keep using it? For a single VM that doesn't need to be up 24/7, I'm willing to bet the performance gains from any virtualization software will be minimal compared to what you're using now.
p.s. And VMware Player is handy if you just want to be able to use a VM and not manage it. You can download a lot of pre-made VM "appliances" and just hit the ground running. For example, there are Nagios and FreeNAS appliances, so you can just go straight to functionality after some brief configuration... don't have to worry about which distro of Linux, which packages, etc. or anything. It's already done for you. To me, these are more proof of concept using VMware Player, though. If I were going to use either in production, I would/could then take that configured VM appliance and insert it into cluster that would be up 24/7.
