Gary's immediate post above is almost correct. Up to this point in time, only Digidesign and M-Audio hardware would work with ProTools. However, Mackie have recently announced they've 'Cracked the code' of ProTools hardware exclusivity and offer an alternative to Digidesign only hardware:

http://www.mackie.com/products/onyxiseries/drivers/

Now, unfortunately for those of us using Mackie Tracktion, it appears that they have concentrated on this compatibility rather than fixing bugs of the Tracktion 3 software. In fact it appears they have basically abandoned the Tracktion product for all intents and purposes.

There are many unhappy customers of Tracktion 3. I never bothered with the upgrade to Tracktion 3 from T2, and glad I didn't.

Gary's suggestion that you post your requirements - in this case, I would recommend that you think about requirements only in the sense that you try to think about what you would like to do from a recording and editing standpoint functionally and write those things down.

For example - I'll give you my functional requirements that I used when I selected my interface:

1. I needed a headphone output so that I could isolate the playback to headphones while I recorded acoustic instruments (this rules out many interfaces right off the bat).
2. I wanted to be able to record up to 2 mics at the same time (stereo mic recording)
3. I wanted to be able to use MIDI recording simultaneous to audio recording.
4. I wanted to be able to play back to speakers using analog outputs
5. I wanted microphone pre-amps with phantom power (rules out many interfaces) because I wanted to record condenser mics without having a separate phantom power source. I also wanted fairly clean headroom on those pre-amps (this was tough to sort out and I don't think I did a good job on this part)
6. I wanted PC/Mac interoperability (I thought I was going to buy a MacBook - but didn't)
7. I wanted low-latency ASIO drivers so that I could play VSTi soft-instruments with real-time feel.
8. I wanted the interface to be portable (cuts out any internal cards right off the bat)
9. I wanted FireWire as the interface bus.
10. I had a budget of $300 max

That was basically it for me. I was able to weed through the countless options down to a fairly small sized set of interfaces based on these functional criteria.

My Songs, photos, writing,etc.
My Sound Cloud