ESXi 5.0 has some additional installation requirements that you should consider before upgrading from a prior version of ESX(i) or before starting a new installation.
Memory – 2098 MB is the stated requirements. Most host systems will have far more than that, but if you’re installing ESXi into a virtual machine you may need to bump up the memory settings for the ESXi VM. Practically if you want to run a few, small nested VMs then you’ll want to allocate at least 3 GB to your ESXi VM.
CPU – there are two requirements to be aware of. First, ESXi does not install to a system with less than 2 CPUs cores. Again this is not an issue for most systems, but if you’re running ESXi VMs then you may need to bump up the CPU (or CPU core) count on the VM to at least 2. If you only have a single CPU core then the installation terminates with the following error: CPU_CORES_ERROR: Your machine has  cpu core(s) which is less than recommended  cpu cores. Second, ESXi relies on the CPU features LAHF and SAHF. These features with added by AMD and Intel to CPUs in 2005 so if you’re running an x64 CPU from 2006 or later then you should have no problems installing ESXi. If your CPU lacks LAHF and SAHF then the installation should fail with a PSOD. Use of the LAHF and SAHF flags are discussed in this white paper – http://web.mit.edu/6.033/www/papers/agesen.pdf.
UEFI – ESXi supports booting from hosts with Unified Extensible Firmware Interface (UEFI). With UEFI systems you can boot from hard drives, CD-ROM drives, or USB devices. However, networking booting or provisioning with VMware Auto Deploy is not possible with ESXi on a UEFI system. Those options require a legacy BIOS firmware. Note: changing the boot type for a host from BIOS to UEFI after the installation of ESXi may result in a boot failure. The host may display the error message: Not a VMware boot bank. Changing a host between BIOS and UEFI is not supported after the installation of ESXi.
Installation LUN size – With support for LUNs larger than 2 TB, ESXi can be installed to a boot device that is larger than 2 TB provided that the storage controller can handle arrays of that size and that the controller is supported for ESXi 5.0.