When you're trying calculate your VCPU:LCPU ratios, you'll want to use the hyperthreaded value of 16. Now, this is assuming that people aren't going to be doing a lot of CPU intensive (*cough* streaming HD video *cough* *cough*) tasks, in which case you'll run out of GHz before you run out of LCPUs.
A common rule of thumb that I've seen is that you can get about 3 VCPUs per LCPU. However, I've seen seen people do good at much higher ratios as well as issues at lower ratios. The way to know for sure is to measure CPU Ready on your virtual machines and make sure that neither of their vCPUs get about 1000 ms on a real-time vCenter graph.
I recently wrote about monitoring CPU Ready and you might find it useful: http://www.vkernel.com/reader/items/configuring-cpu-ready-alarms-vcenter
Let me know if this is what you were looking for or if you have additional questions.
@JonathanKlick | www.vkernel.com