Well I can confirm that I have a Dell PowerEdge T105 running smoothly with ESXi, This system is running from a USB stick thats plugged into the motherboard. No issues!
Spec
Dual Core Opteron 1216 2.4Ghz
2Gb Ram
250Gb SATA Drive
1 Onboard gigabit NIC + 1 3Com 100Mbps NIC
The system at the moment has one VM running Windows Server 2003 Standard RS2, I have yet to install WSUS for some serious performance testing.