I strongly agree with the suggestion of using ESXi; it performs exactly as well as the full-blown ESX product, and it's free.
It can run on consumer-grade hardware (just check the HCL), and will use your hardware much better than any type-2 virtualization solution (i.e. those that run on top of an existing O.S., like VMware Workstation).
The only downside is you will need another computer in order to connect to your host, as it will not have any usable physical console... but it's really worth the additional cost.
Aside from CPU cores and RAM, also be sure to buy as many disks as you can: four VMs spread across four small disks perform dramatically better than the same four VMs residing on a single big one.
If you want to use only one computer and still get the full power of a type-1 hypervisor (and if you can get a licensed copy...), you also have the option of installing Windows Server 2008 R2 on your development computer and enabling the Hyper-V role. Even if it will look as if you were running a hypervisor inside Windows (just like VMware Workstation), the opposite is in fact true, as Hyper-V actually puts Windows inside the hypervisor, allowing it to make full use of the hardware.
Just be sure to not try high-end 3D graphics, as the effect can be quite unpleasant; but if you want to read documentation, send e-mails, check the web... or just actually use your VMs, it will do fine, and you will not need another physical computer.