It'd probably be cheaper to run thin clients and virtualize the whole place, if you don't already have systems purchased. A thin client that works on RDP would work just fine and you can honestly get used ones cheap on eBay. You really don't need to worry too much about the processor speed in the thin clients because all of the processing is done at the server and they're pretty much just rendering video and keyboard/mouse over the network. One multi-core (4+) processor would support several clients, as long as the disk speed is pretty high. A RAID10 setup with SATA 6G drives would probably be a screamer. RAM is cheap enough now that you could load the system up and allocate a ton to each virtual machine. You really wouldn't even need server grade hardware, as long as you stick with quality brands. I'm an Asus fan, but it's not the only brand out there that's worth buying.