depends how many IDE slots are on the motherboard altogether, I have an Epox board with 2 normal IDE slots and two RAID IDE slots with 2 drives attached per slot that gives 4 IDE RAID drives and 4 IDE drives (8 total which could all be hard disks if necessary)..
I use it as a backup mechanism really, I have it configured to mirror drive one onto drive two (RAID 1 Mirror) so if either fails I always have a backup that can be used to run the system from while another disk is installed/etc. There are a number of configurations possible depending on what you realy need RAID for... "Do I REALLY need raid anyway" is the question you should ask yourself and then "should I buy a board with SATA RAID or IDE RAID" is the other,, SATA is the way forward and will add some future proofing for a little extra money.
If I was buying my system today I would definitely choose SATA RAID as the price difference is slim.
Also consider the setup, I have the mirrored drives running on opposite channels so thres always maximum available IDE channel bandwidth; All other drives (non-RAID) are running on the normal IDE channels..
You dont have to use two drives exactly the same (although its a good idea) as raid will simply use the smallest drive size across all drives in the configuration, ie if you had 100 and 120gb drives you would only be able to use 100gb in a mirror configuration, in RAID 0 stripe both drives are combined and you'd get 220GB striped across the two drives but viewable as a single volume (or somewhere near after formatting/etc).