Gather parts to build a system for your intended purpose:

Research your system board & processor & be double or more sure it is supported.  Number of PCIe, PCI should reflect what you want to run. Internal serial HEADER is preferable over DB9 on the back.  Avoid onboard video. DL the manual & check in the settings for VT-d, or "virtualization for directed IO"  Check the Intel ARK to verify your CPU supports VT-d. Similar process for AMD, no first hand exp here sorry. 

Use these links to select known working verified CPU/MB combos :

https://docs.google.com/spreadsheet/ccc?key=0Aqp_xYBwP_Y7dE5EclhtaDdIV09lNWxfODd1alRUTlE#gid=0

http://en.wikipedia.org/wiki/List_of_IOMMU-supporting_hardware

Check the ESXi 5.0 HCL to make sure the nic on your board is supported, or that there is at least a patch that makes it work. If you need to apply a patch, additionally get a nic card that IS on the HCL so you have the connectivity necessary to apply the patch.

Lots of info here:

https://communities.vmware.com/thread/297072?tstart=0

GPU:  ATI HD series are good for general purpose, pass through easy. You need above HD5XXX to run SteamOS, or for crypto currency mining. 

High end NVidia cards work, K4000 tested. 

Low end cards in the PCI slots work, it seems NVidia works better than ATI in this case. 

I used a 2 GB SSD for the ESXi install. You can install ESXi to a USB stick, or even on the Datastore HDD. The SSD's are cheap, boot faster than USB, and best practice is to have your ESXi install separate. 

8 GB ram is about the minimum useful amount. 

Virtual disks can be pre-allocated (faster) or defined as some maximum size that they can grow to.  So you can define 6 500GB virtual disks on a 1TB drive, and it's first come first serve as the disks grow.  Which means you can over-provision your VM's initially, and move them to larger storage when needed.  

Prior to virtualization, datacenters struggled with "server sprawl", and now we have "virtual server sprawl".  Which means since it's so easy to create/copy VM's, they tend to multiply....

So size your storage accordingly.

You can use iSCSi, NFS, or ATA over Ethernet if you get the HBA card form Coraid. 

Unless you build a bootable USB drive with the ESXi install, you will need a cd drive.

I use a Zalman ZM-VE300, which is a 2.5" drive enclosure that allows you to select an ISO file and emulates a USB CDROM drive.  Haven't burned a cd (or coaster) since.  

Keyboard, Video, and Mouse (KVM) matching the number of GPU's, or a KVM switch.

At this point you have an assembled system, VT-d capable CPU & Board, 8+ GB RAM, 2+ ATI GPU's of at least HD5xxx, bootable storage for ESXi and storage for VM's, can boot the ESXi installer from cd/usb/iso, and a supported nic. 

Get ESXi installation ISO from here:

Boot into the bios (or UEFI) and verify:

VT-d is enabled

Execute-disable is enabled

boot order facilitates installing ESXi from your chosen media