Actually, we have two different issues here:
1) RAID
2) The dreaded 2 TB MBR limit.
Let's address them separately
1) RAIDAs everyone has said, Windows does not see the drives as individual drives. It sees it as ONE. BIG. DRIVE. That's what RAID is: one big drive.
RAID 0: Two drives acting like one big one. For simplicity sake, if you had one pr0n video, half of it would be striped to one drive, the other half to the other (actually, it's more complicated than that, but I'm trying to keep it simple. If one drive dies, all the data is gone. Total usable capacity is the size of both drives. So two 80 GB hard drives in RAID 0 = 1 160 GB RAID 0 array.
RAID 1: Two drives acting like clones of each other, but seen as one big drive by Windows. If one drive dies, the other drive has a copy of your data, and you can stick another drive in place of the dead one and rebuild the "mirror." Total usable capacity is the size of one drive. So two 80 GB hard drives in RAID 1 = 1 80 GB RAID 1 array. The other 80 GB is a "cover your ***" drive.
RAID 5: A minimum of three drives acting a lot like RAID 0 (striping your pr0n) and a little bit like RAID 1 (one drive dies, put another one in its place and you're safe--just hope that two drives don't die at the same time or the data's toast). Total usable capacity is the size of all drives MINUS ONE. In this case, four 1 TB hard drives in RAID 5 = 1 3 TB RAID 5 array.
2) 2 TB MBR limit.In this case, we have another problem. MBR disks have a limit of 2 TB. Some RAID controllers will only allow you to make 2 TB arrays.
What do you do about this?
If you're using Windows XP or anything older, not much. You're going to have to partition and make sure one of your partitions don't exceed 2 TB.
If you're using Server 2003, Vista, or Server 2008, then you can format your drive array as a "GPT" disk. GPT disks have the following caveats:
1) Older operating systems can't see them. They'll only see the protective MBR on the disk and read it as an unreadable 2 TB drive, even if it's more than 2 TB.
2) You cannot BOOT to a GPT disk without a special motherboard. Even with that special motherboard, Server 2003 can't boot to a GPT disk. Vista and Server 2008 can, though.
What I mean by "special motherboard" is a motherboard that doesn't have a BIOS. It has what's called an "EFI":
http://en.wikipedia.org/wiki/Extensible_Firmware_InterfaceOnly an EFI can boot to a GPT partition.
...and no, you can't have an MBR partition and a GPT partition on the same drive/array. If your RAID controller allows you to create "virtual disks," then you could create one MBR virtual disk and one GPT virtual disk.