As long as the processor supports Intel VT, it will be able to run XP Mode. Same with your two choices in versions of Windows 7. However, the general consensus is, there's very little reason to go for Ultimate. There are very few features in Ultimate that are not in Professional. I have Ultimate because of TechNet, but if I was buying for myself, Home Premium or Pro would be the answer. Microsoft has a great chart showing the differences on their Windows 7 page. 8 GB of system memory will be great for VMs.
I would also compare the price of building the system yourself through Newegg.com. You might be able to do it cheaper, and you'd have more control over what is in the system. It isn't very hard to build a computer, either. My wife built her last three, until she decided on a laptop.