RAID on most consumer level motherboards is worthless for linux/bsd. it is considered 'fake-raid' or host based software raid.
If you want to utilize FreeNAS and ZFS, you dont need to worry about RAID at the card/motherboard level. You need to focus on just having SATA ports on a card/motherboard that FreeBSD will actually see and use.
CPU selection? Anything newer than Pentium 4? Really it doesnt matter if you have enough RAM and decent SATA ports and NIC.
yeah...just about any system that has any level of HW raid onboard will support JBOD. ZFS likes to take over everything from the array level all the way up to the file system...sometimes to it's own detriment. :)