Well, I have a Dell E1705 with an T7400 processor and 2GB of DDR-667 memory, and I run XP Pro with 2 virtual machines often, and the most limiting factor I run into is RAM. My system does pretty damn good for performance until I load up 2 virtual machines. I'm almost certain that RAM is the limiting factor as I see the swapfile start getting used heavily even when I use just 512MB for each virtual machine. When i'm running just 1 virtual machine doing CPU intensive work quite often I see it best out the system where I quite often have to do the same identical task, sometimes by more than 30%. The system that I'm comparing to is a Pentium 4 3.6Ghz with 2GB of ram. But when I start doing alot of RAM intensive tasks, that speed gets sucked dry. My advice would be to build a modest Core 2 system and put 4GB of ram in that bad boy. When I saw your RAM on your system I was thinking to myself how little ram you are allocating to the VM machines since you've got less than 1GB. I bet you'd see a pretty descent speed increase from your current system if you added more RAM to it and give the VM Machines more ram. I use VMWare Workstation 6.0, and I'd say 6.0 is a bit faster than 5.5 IMO. So, to answer your question I provide 2 answers:
1. Add another 1GB of ram to your machine and give that computer some new life for your VMs.
2. Build a modest Core 2 Duo system with 2GB of ram minimum.
Vmware Workstation 6.0 lets you allocation up to 2 processors to 1 virtual machine, and I can verify that it does in fact work. So having dual core can make everything work alot better since you are very much multitasking with 3 OSes loaded at the same time plus all your software.