With VMs, memory > disk speed > cpu power
My mistake.
So CPU is not the bottleneck when it comes to speed? I usually run one VM and OS X side-by-side. So as long as I get 16 GB of RAM, I shouldn't notice any significant difference in speed?
CPU is generally the least significant factor in VM performance, unless you're hammering the VM. Some people think "more is better" and give their VMs 4 or 6 vCPUs - this is seldom helpful, and in fact is a detriment if you run more than one VM because the hypervisor has to wait until it can schedule time for all of those vCPUs. VMWare generally recommends starting with 1 vCPU and increasing if CPU load gets too high (say > 70%). For a Windows VM with lots of concurrent apps, you might want to start with 2 vCPUs to get decent performance on both the Windows UI and the apps you're running.
In my experience, the top priority is RAM. VMs tend to become really sluggish if they start using disk swap space - an SSD helps enormously, but best to ensure that you've got enough RAM allocated for the VM and avoid memory balooning / oversubscription if possible.
Fast disk storage is the next factor - SSDs have made a huge difference to VM performance. It's generally OK to run the VM guest on the same SSD as the host OS these days, but if you can have two SSDs - one for host, one for guest, it will be better. Hard to do on a laptop, though! Internal SSDs (Sata III, PCIe) are better, but I've had good performance with fast USB 3 enclosures with Sata III SSDs inside them. Thunderbolt would be a bit better.
I upgraded my 8GB i7 MBA 13 to a 16GB rMBP 15 mostly for the VMs - I need to run 2 or 3 large VMs simultaneously and 8GB wasn't doing it for me. I could have upgraded to a rMBP 13, but thought, "hey, lets just get the quad core to get more VM grunt", and I don't regret it (apart from carrying it!)