IMHO: Don't use it as a system drive. lol Keep it totally separate 'if' possible or as a preference. P2P sharing fragments drives very quickly & slows the system down if using the same drive or especially same partition as the system software. LimeWire & most other programs look to the personal user account folder on the system drive as the default location to save files to. So be aware of that.

Are you referring to internal or external HDD? If using an external drive, would need to have a very fast connection between, I'd suggest nothing under firewire as USB can also be limiting in various ways, especially for transmission of certain types of media. Raid sounds very good, perhaps over the top but I knew of several other people who used raid systems for sharing.

Personally I use a separate drive-partition for sharing. Though I do in fact share from a couple of drives (3 partitions.) One partition is an iTunes music library partition (other two partitions from same drive are Windows XP & OSX Applications), other two shared partitions from another drive. One of them I download p2p to & is a general use parititon, the other is more of a static file partition (no work files there.) Thus, only one partition on my downloading drive is being 'both' read from & written to. I use Mac OSX btw. (Notice I keep as much of my read & writing files away from the system. I only keep the applications that demand to be on the OSX system drive & do not save personal files to the system drive which is on a drive on its own. )

Peerless would be a good one to ask about this. Not sure if he will be around over next several days though, he has heavy commitments at his work at moment. He has more experience at p2p sharing of different types than me. Some I haven't even tried as of yet. (I've used gnutella, erm another I started with which is almost obsolete now, direct connect & torrents as the main ones.)