Should work, the ACA-500 doesn't have a limit as it has and IDE driver on board that supports CF-Cards higher than 4GB.
I would use a 2GB-4GB as the primary boot CF-Card, and then the 256GB CF-Card as PC FAT card.
I currently use a 2GB for the OS and a 4GB for the PC FAT card. Was looking at a 128GB but haven't had the need for more space yet.
http://wiki.icomp.de/wiki/ACA500If you really want to make sure you can post your question on EAB on the below post and Jens usually replies there.
http://eab.abime.net/showthread.php?t=65087&page=78
