What are the rules for hyperthreaded CPUs?

Hi All,

My host has an i7-960 processor (4 cores, 8 hypterthreaded cores.) /proc/cpuinfo on my host shows 8 processors.

On my Fedora Core 16 guest, /proc/cpuinfo shows 4 processors.

Question: what is the maximum number of processors I can assign to my guest? 4 or 8? What are the rules for Hyperthreading?

Many thanks,
-T