I like the idea about making the XP machine into a VM. From the sounds of it you don't use it all that much. Unless your XP box is doing intense graphical stuff, I think this is the best idea. You can use your XP computer alongside your Win7 computer, and you don't have to worry about a second computer. If you are doing lots of graphics, I would choose the dual boot option.
If you do make a VM and you have problems with the shares, make sure you look at the network settings. I don't have any VM's on this computer, but there are usually options to share an internet connection with your host computer only, use the same IP as the host computer, or assign the VM with its own IP. Make sure you choose the last one, it will act just like a normal computer on your workgroup/domain.
For a server computer, I would choose your 2nd best computer, and make that a dedicated server. If you can get a copy of Windows Server, it has all the components you should need for a server computer. I would not put a server in a VM, because it would have to be running all the time, and if you ever have to reboot the main OS, the server shuts down as well. Another cool thing about servers, is that you can enable RDP, and just leave it run in a corner with a power cord, and internet cable. Then just use it through remote desktop connection.