Avid and FCP use different codecs!
I would go for Xserve Raid and Xserve Server: The raid is the best on Raid5 with brilliant speed (Fibrechannel) and works perfect with FCP and Mac. I know it’s expensive, but if you just think of going for a cheaper solution and having problems with it, it is worth every cent. The XRaid has got 14 slots with a maximum of 750GB per slot (Raid5 takes off some storage, but it is safe)
Rainer