HP Workstation xw8600
Proc: Intel Xeon 54xx series processor (Quad Core) (1 of 2 sockets filled)
RAM: 2048 MB (2x 1024GB DIMMs) DDR2 PC2-5300 FB
NIC(s): 2x onboard NICs 10/100/1000 NetXtreme BCM5755 Gigabit Ethernet
Storage: onboard SATA 631xESB/632xESB SATA Storage Controller RAID; also has SAS RAID controller available
Hard drive: 1x 160 GB SATA hard drive
Supports Intel VT-x: Yes!

Installed easily out of the box without any major modifications.
BIOS setting to enable 64-bit virtualization (Intel VT-x)

F10 (BIOS)
- Security | OS Security | Intel Virtualization Technology = "Enabled"

I've been running VMware ESXi 3.5 on this for the past six months without any major issues. Guest OSes loaded/tested so far: MS Windows XP, RHEL 4.x, Ubuntu 7.x, OpenFiler, CentOS 5.x. Started out on VMware ESXi 3.5 U1 and now at VMware ESXi 3.5 U4.

I like the box so much, I picked up one on the used market for home
This particular model is nice for ESX/ESXi because the workstation supports up to 128 GB of RAM and two Quad Core Xeon processors. It has a built-in controller for SATA (6-ports) plus another built-in controller for SAS (8-ports), with space for at least four drives internally, plus another another five if you use a 3x5 SATA backplane in the 3x 5.25" bays.

I hope to try out ESXi 4.0 on my xw8600 at home

That's a nice option for a dual CPU MB. And it have 7 PCI* slots?

The motherboard on the HP xw8600 workstation has seven slots total:

2 PCI-e x16 Gen2 Graphics
2 PCI-e (x8 mechanically, x4 electrically)
1 PCI-e x8 (switchable as x1 or x8)
1 PCI-X 133 MHz
1 PCI

PCI-e = PCI Express

And two physical sockets for processor, each of which can run an Intel XEON Dual Core or Quad Core processor. Plus it has both a 6-port SATA RAID controller, an 8-port SAS/SATA controller, and two Gigabit Ethernet ports on the motherboard.

If one had the finances to run it fully loaded... (2x Xeon Quad Core X5482/ 3.20 GHz, FSB 1600 MHz, 16x 4GB PC2-5300 FB DIMMs [128 GB RAM], one or two quad-port Gigabit NICs + 2 Gigabit ports onboard), and 10x 1 TB SATA drives (using 3x5 backplane in top and 5 internal bays), that would be one sweet ESX host

Even without the four riser boards that lets it take 16x DIMMs, it can take 8x 4GB DIMMs, and have 8 VMs that would have 1x vCPU of 3.2 GHz, 4 GB RAM, 1 TB disk, and 1 Gigabit NIC dedicated, consuming less than 1080W of power max. (max 135W power per VM).