Ok, short answer: there will be a speed increase in writing, but not much of one, on the same bus. On the cd-rom bus, there will be a significant performance increase, but the two buses run at different speeds, which can cause problems at boot (I have a RAID with one drive on a PCI card channel and the other on the CD bus; it only boots about half the time, but it works great when it does).
Your best bet is to buy a PCI drive card for $50 and run the drive on separate buses on the card. I would do that, but I have a 200g drive in my Sawtooth as well, which needs the card to be recognized. That will be the most stable and fastest solution.