ExpressCard is good. I believe it's a much more tightly integrated path with the CPU and motherboard than USB/Firewire.

Guess you can't use expresscard soundcards with a desktop, though? Potentially limits its flexibility. I guess you can buy breakout boxes for PCI expresscard though.

RME Fireface UC
 

Same price as FF400 but supposed to have similar latency as PCi on decently speced PC. It is USB. Apparently you can get lower latency than FireWire because FireWire has built in latency.