Hardware requirements for a virtual server

Here is a small list of requirements. Hope this helps you.

Minimum CPU Speed
550 MHz
Number of Processors
All physical processors supported by the host operating system.

Recommended CPU Speed
1.0 GHz or higher

Processor Information
Intel Celeron, Pentium III, Pentium 4, Xeon; AMD Opteron, Athlon, Athlon 64, Athlon X2, Sempron, Duron Core DUO, Duron Core 2 DUO processor

Supported Host Operating Systems

Microsoft Windows Server 2003 Standard Edition, Enterprise Edition, or Datacenter Edition or later

Windows Server 2003 Standard x64 Edition, Enterprise x64 Edition, Datacenter x64 Edition or later versions

Windows Small Business Server 2003 Standard Edition or Premium Edition

Windows XP Professional Service Pack 2 or later (for non-production use only).

Minimum RAM
256 MB (additional memory needed for each guest operating system)

Required Available Hard-disk Space
2 GB (additional disk space needed for each guest operating system)

Recommended Monitor
Super VGA (800?600) or higher resolution monitor recommended; VGA or hardware that supports console redirection required

Keyboard and Microsoft Mouse or compatible pointing device, or hardware that supports console redirection required.

I'm in the process of building a similar setup as yours. We've selected the gear and now are in the ording phase. We'll be building starting next week and will be deploying very soon.

We chose HP DL380 G5 with a pair of the 2.33GHz Quad Core Xeon's, and 8GB RAM, possibly more RAM. It's not just about the number of guest O/S' you're hosting, but what will they be doing? For example, in our case we're building 2 physicals to host about 8 virtuals, but 1 of the virtuals are SQL Server and another is Exchange; we'll put those 2 on separate physicals, and spread the rest of the virtuals amongst the 2; others are DC, FS, Webserver, Term Servers, App servers, etc. -- relatively light duty compared to SQL or Exchange.

My thoughts are, and this is my first VM project so I'm learning as I'm going, but based on my research and my own general experience at this point ...
a) proc -- from what I understand, VMware is licensed per CPU socket, but is pretty efficient at managing cores, so the more cores you can throw into a socket, the more processing bang you can get for your licensing buck. So if budget says you can get 2-duo cores or 1 quad core, I'd go with the quad core, knowing I can slap in a second quad when I have a little more budget again, and double my processing power, woo-hoo!

b) ram -- consider what your guest O/S are doing, sum up all the RAM that you would give to your guests if they were on their own, and come as close as budget will allow to that. Sure you're gaining some efficiency going to the virtuals, but at the same time, you're buying one physical instead of 4 or 5, so don't short-change your RAM. RAM is reasonably priced these days and you do NOT want your SQL server running out of RAM.
c) disk -- as with the RAM, sum up what you would need separately, but differing from RAM, I would err to the side of over-estimating, plus add in a growth percentage. Unlike RAM and proc, disk isn't always as easy to increase. In our case, we're using SAN (albeit a baby SAN); it'll make the disk easier to grow or change as the company changes (hopefully growing), as you can do some wonderful things with SAN. Disk itself is relatively inexpensive these days, but there is an investment cost in implementing SAN.

Again, not tried and true advice, but being in a similar situation as yours, this is where my thoughts are at the moment. Hope it helps.