24x 1" hot swap sata server chassis

Hi, long time fan of the site, first post. Recently I've been given the task to find a low cost high capacity file server. I work for a school district and we're hoping to get at least 5TB of storage, but we need it to be able to sustain sudden peaks of high loads with moderately low latency. We have around 200 teachers and few thousand students, and currently teachers get 200MB and students get 50MB of network storage. We have a total of 900GB... Now usually we go with Dell for our computer needs since we can get ridiculous discounts, but their file servers just don't cut it for their low end, and the next step up hits well over $20K. So now we're looking into me building one! My solution is go with two socket F dual core AMD Opterons, two Areca ARC-1230, and 24 7200RPM 320GB SATA drives. All RAID will be handled with the Linux md driver and I already have that all sorted out how I want it (combination of RAID 6 and RAID1+0 for separate partitions for the operating system, plus two hot spares). The reason for using software RAID is to become hardware independent, i.e. the RAID card doesn't matter, as long as the HDD's are hooked up. Besides, XFS can crank out some ridiculous speeds; I should get an excess of 600MB/s after tweaking, mostly limited by the cards connecting the drives.

Now, my question for you guys is two things. First one is I'm still not totally sure on which motherboard to use. I was thinking of getting this one. The decisions for motherboards are pretty hard in this area, but Tyan usually makes some good stuff, albeit picky at times. Cost isn't an issue with the motherboard. The second question is does anyone have any experience with a good 24x 1" hot swap server chassis? There's a number of them to pick from, but prices range from $1700 - $3000. Will certain backplanes effect performance at all, or is my main worry with power supplies (most have tripple + 1 redundant in the 700-950W ballpark) and ease of use? Size isn't an issue, but I would like to stay on the lower cost if possible.

If anyone else has any other suggestions I'd love to hear them!