G'day MadMick,
One of the great things about Open-E is that it is pretty compatible with most hardware.
The specs that Symm have listed are a good place to start.
It depends on what your needs are, and whether you are looking to try out Open-E or build a serious NAS to store TB's of data.

A whitebox system will work fine, we have them running on everything from converted HP Workstations/Intel MB, or TyanMB/Areca Controller, to Intel MC212/Adaptec 5805 servers.

Also, make sure that you have current firmware for your Bios/RAID and disks.
We have had a few issues with Adaptec and Seagate drives that are a real pain until you update all of the firmware levels.
But to start you can simple use the Open-E software RAID and the MB controller.

Good luck.

Rgds Ben