The fastest hard drive setup I can think of is external SATAII drives connected through an adaptor plugged into the expresscard/34 slot. this is a full speed PCI-X interface. The reported speed of this interface is up to 3Gig/Sec. Much faster than even the firewire 800 port. Havings two drives setup in a raid array in an enclosure that has a SATAII port would be insane!
The Lacie big disk with serial ATAII port and an expresscard/34 Serial ATAII adaptor might be AU$300.
Do you need this speed? Probably not. I've just bought the Santa Rosa MBPro with 160g/5400rpm drive. I didn't want to pay the extra and wait for the 7200rpm drive because I've been using a 5400rpm drive in my 12"G4 powerbook and it's never the hard drive access that slows me down. It's always processor. If I find with this machine that the hard drive access is too slow, I'll replace the 5400rpm drive a little later with a faster larger (CHEAPER!) drive and use the internal in an external enclosure.
I also went for the 2Gig of RAM for much the same reason.
the other thing the expresscard/34 slot could be used for is a solid state drive, now that's something I might do.