I'd want a Sandy- or Ivy-bridge Core i5 or i7, and definitely more than 4GB RAM - 6-8GB is great for Windows 7, more if you regularly run lots of VMs.

I don't know if this matters to you, but some manufacturers disable Intel VT in the BIOS with no way to re-enable short of reflashing the BIOS, even though the processor does in fact support VT. You don't need VT to virtualize 32-bit OSes but virtualizing 64-bit OSes needs VT support. A lot of "consumer" laptops seem to have this problem here.

You might also want fast I/O - USB3 or Thunderbolt (available on Ivy Bridge motherboards).