Building a computer yourself always beats those pre-built systems. I would recommend going with the new 600 CPUs from Intel for anything else than a a hardcore gaming rig. A very nice main board is the Asus P5AD2-E Premium, with it's built in HD sound, a huge bundle and room for all the storage units you want - six IDE and a bunch of S-ATA. Also it has built in WLAN and two LAN ports. Combined with a CoolerMaster Stacker case, with room for all those units, and preferably water cooling (quiet, efficient) you'll have a system that eats Alienware for lunch, at probably half the price.
I'm not big on RAID myself, I prefer to have a full backup in another case that's not connected to the power grid (mine's an MP3 computer in the car), since power spikes can kill all disks if they're in the same case. 200 or 250 gig disks would be the way to go when you want as many gig for your money as possible at the moment.