My company needs to test a windows application across different operating systems. 
So we would like to have different operating systems that resides in a single machine. 
Among those VMs, I have heard about
- Virtual PC by Microsoft 
 - VMware Workstation 
 
Are there any better ones than those mentioned above? 
My questions are 
- Which is the best VM available now? 
 - Can one simulate a 64-bit OS on a 32-bit machine using these VMs? 
 - Which will be a good configuration for machines that installs VM? 
 - Are there any better alternatives? 
 
Regards, 
Naveen
P.S: I have also posted the same question on stackoverflow since i couldn't figure out exactly where this belongs to.