There is no upgrade Path to ESX from ESXi, both are bare metal Operating Systems. Even with the release pending of vSphere your upgrade to the console version of ESX would be a rebuild if running ESXi. Yes, ESX(i) supports multiple CPU's. Here are a few documents that may be worth while
What might not be completely clear in regards with the inplace upgrade and that there's indeed no upgrade path from ESXi to ESX is that it is still very easy to do.
You can do a reinstall and keep your VMFS storage where the virtual machines are stored. Of course you would want to make a backup of your VMs before you take a drastic step such as that. (It never hurts to be on the safe side)
A reinstall only takes like 15 minutes, after that it is a matter of reregistering your VMs and you are back up and running.
So it is not a re-install as you would have with a normal operating system, it takes much less time to complete.
Visit the VMware developers wiki at http://www.vi-toolkit.com
Yes ESXi supports dual CPUs - as others have posted there is really no upgrade path since both are bare metal hypervisors - the license you get for ESX is usable with ESXi - so if you do not want to reinstall your server you can simply point the ESXi server at the new license -
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