Increase number of CPUs from 1

I had to reduce my CPUs to 1 to get P5 to accept my XP 3 VM, but now the selection for number of CPUs is greyed out - only the 1 is selected. How do I get back to multiple CPUs and what is the optimal number. I am running 10.6.2 and have have a MacPro with dual quad core Xeon cpus.

 

you have to shut down guest os and then it wont be greyed out.

 

Thanks, that works.