Just to throw in a wild-card .....
Why not run PT on a Windows machine? I've been running an HD4 rig on Windows XP for something over 18months and it is rock solid (currently at PT HD 7.1). It frees you up from the limitations of Mac hardware, PCI-Express etc., and means you can configure a very powerful machine for probably slightly less cash, with plenty of PCI slots and hard drive bays (4 HD Accel cards all fit in the box here, along with up to 8 hard drives). Overall it should save you some money which you can spend on interfaces, converters, w/clock etc.
Incidentally I also run a Mac G5 (2x2GHz) here, but personally wouldn't want PT on it - I use it for Logic. If you already have good converters, all you need is a 192 Digital IO to give you your 16 channels. I have 4 Digital IO's here plus a 192 and a 96i. All are great interfaces - the 192 converters are perfect for hardware inserts, and I use a Universal Audio 2192 as the main AD/DA on the mix buss.
Lots of negative feeling about Digi' and PT around here at the moment, but I have to say I have never looked back. With good plugins, this system is very hard to beat.