Your lab sounds similar to ours though we also have a few stand alone machines running various OSes. We use Gentoo for our test events with various tools.

I have a pretty beefy machine with vmware workstation on it that I use for most of my testing. I also have a couple crash and burn machines around for misc testing.

I just got a copy of it recently. It's a pretty good one.

I just built a new machine a 2.8 Core2duo with 4GB of RAM running linux and vmware server...runs like a champ

Virtualbox is great with one exception, the networking.  If you are interested in having your own network and putting all of the virtual boxes on the network through NAT, then Virtualbox is cake to setup.  The one beef that I have is that it sucks to have to create individual bridge adapters and then more individual adapters for each box linked with the bridge device.  Then if you want DHCP on a host only network, you need to set that up yourself.  In comparison to VMware, virtualbox needs to fix that.  As far as speed, I think Virtualbox is much faster and seems to have a smaller memory footprint.  For my linux boxes, I use virtualbox as my VM architecture since the hardware support seems great.

As far as lab boxes,  I have 2 linux boxes, one a quad core 4g ram and one a dual core with 2g of ram that I use.  I use ESX server on them and have a host only network on each with a small linux box I use as a gateway server and allow or disallow bridging when I need it to bridge the host only network to the real word for software downloads etc.  I have a number of other boxes that I use in bridged mode to do testing with.  This setup is nice in that there are some boxes that I really REALLY don't want on the network, such as XP sp0 boxes, and others which require network access. 

Wow thats a hefty lab, have you just collected that much gear over time?