To be honest, I use seperate drives for each operation... I have a couple of setups that are RAIDing, but they are more for large space rather than speed.
For example, my main PC right now, has a basic setup of :-
SATA 0 = Seagate 80GB
C: - 8GB ( Windaz )
T: - 68GB ( ISO Images & Stuff )
SATA 1 = Seagate 120GB
D: - Apps & Games
SATA 2 + 3 ( Seagate 250GB x 2 Striped )
E: = Media
IDE 0 = R: NEC 3550 DVDRW
IDE 1 = V: NEC 3540 DVDRW ( Better than the 3550 cos it works properly )
IDE 2 = W: Pioneer 110
IDE 3 = L: LiteOn LiteScribe DVDRW
ITE 8212 Card
IDE 4 =
F: = Maxtor 40GB
IDE 6 =
O: = ZIP250 ( God knows why )
I use the Maxtor cos its seemingly the most reliable model of drive I have ever come across - its been torrenting 24/7 for almost 2 years now and its still 100% perfect, so I wont use any other.
Now, in terms of speed, I can burn 4 DVDs, from say Drive E:, defrag C: and F: using O&O, Play a movie to my TV and play a game of Quake 4 and I have absolutely no glitches of issues at all, and the levels take just a few seconds to load.
As long as you dont try to do everythign through one line, you should be ok.
Hell, My NF7S and Barton 3200 does everything just as well.
The only system I have that fails to do lots of thigns at once, is my P4 3.6HT and thats supposed to be able to, but thats Intel junk for you.