Firewire is not a dead end in Mac land. I am confused where you're coming from on this one. Competent Firewire enclosures can be had for not much more money than USB 2.0. FW is faster than USB 2.0, and it has a more intelligently designed bus. Plus, you can find peripherals that use both, thus having total compatibility.
SCSI is almost obsolete for external drives, but largely because Firewire and USB 2.0 has pushed it out of that market. It's now mainly a niche product for servers.
To answer the OP's question, I would either add a Firewire card. Or if that's not in your budget, go SCSI. USB 1.1 is painfully slow for data transfer and should only be used for low speed periperals, and maybe the occasional scanner/printer.
- John