Well, you could be pretty flexible in terms of audio interfaces. Basically, you need a 4 input and 2 output interface for your needs right now.
So, some good Firewire based interfaces to look at would be:
- Presonus Firestudio Mobile (10 inputs, 6 outputs - 2 mic preamps, very reasonable price)
- Mackie Onyx (More than you need, reasonable price - never used one though)
- MOTU Ultralite (10 inputs, 14 outputs, built-in effects, 2 mic preamps, good stuff)
- Focusrite Saffire Pro 14 (8 inputs, 6 outputs, 2 mic preamps included, great price - and there's the Pro 24 for more ins and outs, etc)
- RME Fireface 400 (8 inputs and outputs - 2 mic preamps - very good quality - but pricey)
If price was an issue I'd look at Presonus or Focusrite... but the RME mic preamps are probably the best (useful when you consider recording audio from a microphone).
I haven't listed whether they include MIDI In and Out as you've already got the fastlane which should suffice.
Hope this helps