vSphere IS ESXi (or ESX) with the manangement add-ons...
If you're talking about vCenter, then just make a VM on one of the hosts (on shared storage) and then start configuring it there. I've done this more than a few times with great success... Having the vCenter Server as a VM has several advantages, not the least of which is being able to vMotion if the host it's actually on goes dark.
What kind of SAN are you using for the VM's?? What are you installing ESXi onto (on the physical hosts)?? Even though you can install ESXi to flash drives, I still prefer to install to a pair of SAS drives in a RAID 1 configuration. You don't need very large, or fast (anything over 10k rpm and you're golden) to do the job. You could do it with SATA drives, but you're better off with SAS (higher reliability numbers). I also like the RAID 1 configuration in case of drive failure. Just more resiliency to failure, or one less single point of failure...
BTW, I would also use an actual SQL server for the database for vCenter and it's parts. IMO, far better than using SQL Express that's bundled with the installer. Sure, you CAN go with SQL Express, but you'll have more management options under full SQL... Make the SQL Server as either another VM (so you'll make two VM's to start with) or use an existing SQL server (for now)... If you use an existing SQL server, I would plan to migrate to a virtual one as soon as you can.
