my QPS x24x10x40 works fine with toast titanium for os x, disc burner, and iTunes. It cost $200 from outpost.com back in December.
other companies have x40 burners now. that's very fast. Wal-Mart sells these, but they're internal. The external ones are USB 2 interface.
You could pick up an internal one ($140 or so) and slap it in a firewire enclosure (which might run you $90+ bucks, depending on what and from whom you buy). However, I don't know if these drives would work with iTunes or Disc Burner. Toast Titinium usually does a quality job of supporting the plethora of burners out there.
The x24 drives should be coming down if the x40 are out in full force, which they seem to be. You might be able to pick up an external one for cheap, especially on eBay.