Wait - are you building this for home use or car use? Cause if you're building this for your car, then you just backed over a kitten All of this hardware is too valuable to make building a car-PC worthwhile. You'd be better off spending $100 combined for a Celly 650, mobo & 256mb PC133 RAM. Plus running RAID in your car is a frivilous idea - you'll just break 2 drives instead of only one.
As for home PC - well hell, anything goes these days. My only posing to you is this: What do you need a RAID for? What information do you need to keep on hand immediately that's too important for you to backup onto CD? (mind you, RAID won't stop an mp3-killing virus... only regular backups will)
If you're using RAID for performance, then you might be better off getting more RAM first. However that really raises bigger problems (using RAID for performance) because then I'd have to ask why you're not going with a DDR SDRAM mobo or RDRAM mobo, and a P4 1.4 or greater.
And on that note, Why not go for a P4 1.4 & mobo? Fry's has been having a P4 1.4 & mobo bundle for $150 every weekend for the past month. Ordering online would be even cheaper (sic)