Just use Converter to import the VM you had created under VMware Server to ESXi.
VMware Server is fine when you're in a pinch or for environments wherein you can't afford a dedicated server to host Virtual Machines. There are many reasons not to run VMs on VMware Server, but it does work, despite having to share resources (and frequently patch your host if you're running Windows).
Running VMs on a dedicated ESX/ESXi host is the recommended solution (so long as your hardware meets the VMware HCL as well as your CPU/Memory/Storage requirements for running your VMs). You can either install ESXi on your local disk or install it on a 1GB USB drive (
http://blog.mechanised.com/2008/07/how-to-create-your-own-bootable-esxi.html).
While ESXi is free, you'll only be able to manage each ESXi host individually. If you require features such as VMotion, HA, DRS, etc, then you'll be looking at VMware's Virtual Infrastructure (which can be a premium cost that requires additional hardware and infrastructure). VMware does have solutions that fit your budget (VMware Server, ESXi, VI) and allow you to expand as your virtual environment grows. It just comes down to what you need and what you can afford.
