Found this (http://zi0r.com/2009/11/22/esxi-on-a-dell-sc1420.html), and it was helpful for my particular situation:
Dell SC1420’s appear to limit CPUID information available to the OS and do not appear to present a BIOS option to the end user to enable/disable the ‘feature’
In order to get ESXi to install/boot, you need to do a couple of things:
1. When the ESXi bootloader appears, hit tab.
2. Find vmkboot.gz and append nocheckCPUIDLimit, before the -
3. Find vmkernel.gz and also append nocheckCPUIDLimit, before the -
4. Hit enter and the ESXi installation will continue.
5. After ESXi has been installed, it will reboot, you’ll then need to press shift-o to access advanced options, simply type in nocheckCPUIDLimit in order to get it to boot for the first time.
6. ESXi should boot for the first time, connect to it with vSphere and goto Configuration→Advanced settings.
7. Navigate to VMKernel→Boot and locate VMkernel.Boot.checkCPUIDLimit and untick the box.