Welcome to the VMware Community forums. They aren't supported but you can check ultimatewhitebox.com or vm-help.com to see if others have had success installing ESXi onto them.
We have ESXi 4.0 running on Optiplex 755 with a Intel Core2Duo E6850 @ 3.00GHz 4GB RAM. Work fine.
Are you having a problem with the LSI Logic SCSI showing up in the "safely remove hardware" icon in the system tray of your Windows 2008 Servers like I am? The NIC was showing up too, but I was able to fix it with a workaround I found on the VMware forums. I'm also seeing a device that doesn't have a driver installed inside my virtual machines... I'm guessing it's related to the AMTSOL / LMS Drivers. If you have any insight into these problems your help is much appreciated.