I still like Dell's but just about anyone will work, just dont be cheap on memory, CPU speed and HD size... VM's will suck up a lot if you are using them and the CPU's you listed it would be a good idea to have one or two running them
Also, like someone said above, don't cheap out on getting an SSD instead of a standard HD either. SSD is so much faster, and your VM will lag a lot without it. I have a 1TB SSD (960GB, because rounding?), and it is 80% full.