I forgot about ESXi 4 being 64bit. Yeah ESXi 3.5 will work. I have Zimbra running on ESX 3.5 at the office for a few users. The ESXi 3.5 specific to Dell should have the OEM CIM providers to talk with the OpenManage software. Not sure if it is a deal breaker or not for you. I like having the software to manage the RAID array, etc.
If you try out the OpenManage software make sure the CIM providers are enabled. Open the VMware Client under the host configuration tab. Click advanced settings on the bottom left. Under the Misc section you want to make sure 1 is set for Misc.CimOemProvidersEnabled |