Is your server hardware capable of running vSphere?
Eric Siebert wrote a nice article about how ready your hardware is for vSphere
With VI3 you could run on either 32-bit or 64-bit server hardware but vSphere requires 64-bit hardware because the VMkernel and Service Console are now 64-bit. I did a tip a while ago for searchvmware.com that talks about this in detail and shows how you can test your hardware to see if the CPU supports 64-bit and has some of the other CPU features that vSphere needs for certain features. To briefly summarize that tip:
See the rest of the article over here