I don't know much about Dell servers, however most vendors provide a hardware tests CDROM. If you have one, please run a complete HW test. If you don't have one you could try to boot a Linux Live CDROM to see if this works.
Also check the HCL, I saw one of the Dell 2950 (2950-III with Intel 51xx CPU's) is not supported for 4.1.
I ran the dell diagnostics and everything came back passed except for a power supply redundancy failure. Would that power supply affect ESXi?
Have you upgraded the firmware to last version?
Intel VT is enabled in the BIOS?