Your right, hardware support definitely is not as ubiquitous as it is with Workstation. That was the primary motivator for suggesting the purchase of a new server class motherboard, a close second was the extra memory capacity you gain with those boards. VMware has some other very cool options coming in 2009 with upgrades to the Workstation product line that will go a long way to reduce the overhead you pay for in having a host OS under all your virtual machines. You'll also see a significant performance increase as the VMs get closer to the physical hardware. I don't know if that is something I can be more specific about though since much of what I know about it was learned under NDA.

They have ways to import existing VMs and you can still have clones, snapshots, etc.

Now that VMware has released ESXi if I had your setup I'd buy a motherboard that works with your processor (and buy another cpu if your not running dual yet) that supports more than 4 GB ram and put at least 16 GB in it. You can do that today for under $1000 if you buy a super high end server motherboard. Then load it up with the high speed sata disks you already have, install VMware's ESXi on it (bare-metal) and load your VMs off of that. Major benefits of ESXi over VMWare or Virtual PC is first you don't have the overhead of a full OS. Second, ESXi merges similar pages in memory so if you run 6 VMs all running XP you only pay the memory overhead for the XP OS once. Third, and probably most important, you get memory overcommit which allows you to assign each of your VMs more ram than you physically have in the system (to a point) since ESXi is smart enough to only allocate what is being used by the running VM. You'll also see much better usage of your quad-core and I believe ESXi supports cpu affinity as well for individual VMs. Your basically getting the same capabilities we use today in the data center at home (for free!).