Firewire is a good way to go. I have my program data on the internal drive and only my Giga sound data on my firewire drive. You'll find that they are more expensive than internal type drives but they perform very well. I havn't pushed my drive to the limit in terms of polyphony so it depends on how many tracks you want to run at the same time from Giga. The big advantage with firewire is that you can daisy chain up to 63 drives together so expanding outside your computer box will solve your IDE problem.
Although I've a couple of spare SATA heads on motherboard free, I'm all out of physical space with current 3 IDE drives. Plus the heat generated by the 3 internal drives is starting to get excessive I feel.
I think an external drive, whilst not as fast as SATA, would be a healthier choice overall.
Just wanted to confirm there were no performance issues.
I've run both external Firewire and USB2 drives, and both have worked just fine even when pushed hard- at least with Giga 2 and 16-bit samples. But if you already have SATA on your motherboard, another option would be to buy a SATA drive and a case, say, like this: