I'm always very strigent with memory on my VM's. Using Windows task manager and performance monitor you should be able to determine your VM's memory requirements. Additionally in Virtual Center if you edit the settings of a VM you can specify on the Resources tab shares, reservations and limits. That way certain VM's have higher priority of physical memory then others. If you do not specify a reservation for a VM then ESX automatically creates a VSWP memory file for each VM. So if you give a VM 1GB of memory and do not set a reservation then a 1GB swap file will be created on the ESX server for that VM tying up valuable disk space. I usually create reservations equal to the memory a VM has assigned to it for my important VM's. and specify a reservation of only half the memory for less important VM's.
Check out this white paper...
http://www.vmware.com/pdf/esx3_memory.pdf
And also this....
http://www.microsoft.com/technet/prodtechnol/windows2000serv/tips/w2mem.mspx
