just a quick post, i've recently had to install ESXi onto some new Dell server (R720 and the 1u equivalent) for some clients. I normally download the latest release from VMware and burn it to CD before i go to site to save some time, but this has caught me out with the new Dell Servers.
What you will find is that when the ESXi disk starts to boot you get an error message stating that there are no network cards in the server (even though i had on this occasion 2x quad port broadcom NIC's). What you need to do is go to Dell's website, enter in the server tag for the server you have purchased and got to the enterprise solutions part of the support and drivers section. There you can download a Dell customized version of ESXi which includes all the dell drivers you need to successfully install ESXi
Hope this helps and thanks for reading.