Hmm, bunch of answers/responses to the posts here heh.
One thing to watch out for when using the 75xx/85xx series 3ware cards with large drives is the fact that they don't support arrays with sizes larger than about 2 TB per card. So your choice of one of the new 9000 series cards is a good one..
I guess the 2 TB limit issue would only happen when using the 12 port cards with 250 GB drives, or much more easily when using the upcoming 400 GB drives.
One thing with the 9000 series cards and their onboard cache though; the battery backup that lets the cache keep its data intact if it doesn't get a chance to write it out to disc in case of a power failure is apparently not yet available. A UPS would definitely be advisable for a system of this sort anyways.
RAID5 is a (Capacity/Drive Size)+1 solution, regardless of the size of the array and the drives in it, virtual or physical, all have to be the same size. You CAN use part of a 250 GB drive in say an array of 180 GB drives, you'll just only be using the first 180GB of the 250 (and a lot of controllers I bet don't support this either, though most software RAID solutions do). So (8*400)-400 = 2800 GB, and (400*6)-400 = 2000 GB.
With regards to 100mbit.. and user count, totally depends on what the heck the NIC is actually serving packets out for. For a webserver serving mostly text mixed with some images, 100mbit is plenty; for a server that is serving out DV25 video that needs to be played backwards/forwards/slowmo flawlessly etc... try 2 clients on a 100 mbit line tops.