I don't think I can give you a specific recommendation because the best solution is always
a balance between reliability, convenience, cost, performance, et. al. and I don't quite know what is suited
for your case.
I will say the following -- if you're going to be using commodity "desktop" style technology components,
choose DDR2 PC2-6400 or PC2-8000 RAM, and get 8GB (4x2GB DIMMS) of it. RAM quantity and speed
is extremely beneficial for virtual machine usage like yours, so it pays to maximize the amount of RAM.
For the current DDR2 unbuffered memory prices, I couldn't imagine setting up a VM system with less than
this sort / amount of RAM.
Then get something like a Q9450 or Q6600 CPU; the Q9450 has the best VM technology and is probably the
best choice since your needs emphasize VM usage. The Q6600 has decent previous generation VM support,
and for its current price of around $180 or so, you can't beat it for pure price/performance if you don't think
you'll get much benefit from the slightly newer CPU based VM technology in the Q9450.
If you're going to build the system out of server class components, look for registered FB DIMMs, and get
8GB to 16GB of the RAM since for the multi-OS multi-application VM needs you have, the higher amounts of
RAM will likely be very beneficial. I'd guess one of the newer server class AMD CPUs or the newer generation
of 4-core XEONs would be fine for you. Get some specific VM benchmarks and choose the CPU accordingly,
but pick one with the best hardware VM support that is likely to be relevant to your uses.
I'm not sure if VMWARE is even fully using the latest CPU based VM hardware support in their current
production or beta versions; if they are not, I can only assume they'll do so within a year or so in an upcoming
version. IIRC the newer VM extensions better support virtualizing DMA oriented hardware devices so that
the I/O memory can be virtualized and managed by the hypervisor much more efficiently. If you're not
virtualizing guest OSs and applications that benefit from having advanced I/O devices supported and controlled
by the guest OS/application, though, maybe the older Q6600/X3220 style VM extensions may be entirely
suitable for your needs. Virtualizing advanced GPUs (which you don't need), USB devices, advanced
networking or disk controllers may (eventually) be best done with the newer VM extensions. Just using
basic GPU technology and networking / storage handled only by the VM host, though, doesn't require much
in the way of CPU VM support beyond the basic level many of the last couple of generations of
Xeon X32xx / Q9450 / Q6600 / Opteron / Phenom supports.
If you want to have high availability / run-times of the system in excess of 2-4 weeks between reboots,
I'd consider using server class fully buffered *ECC* based RAM. Without RAM ECC you'll be
much more vulnerable to RAM data corruption due to random and uncontrollable glitches which eventually
over a period of weeks of constant running is not unlikely to corrupt your data and crash your software.
If you can afford to reboot the PC every day or two, and don't consider the data integrity of paramount
importance, using desktop class non-ECC non-buffered non-registered RAM and consumer oriented
motherboards isn't an unreasonable compromise to save a couple of hundred dollars on the RAM and
motherboard.