Unfortunately none of these interfaces looks like the right one for me. The Presonus Firebox and the Terratec Phase x24 could be ok, but I was rather looking for something like the RME Fireface 400 - To put it differently, I'm not sure if these two cards have the same sound quality and reliability as RME or MOTU ones. If anyone use them, please tell me about the results you get.
I'm very happy with my Terratec Phase 24FW but I can't compare it to a RME because I never owned one, so I can't tell you if it's as good...
The main problem is to find a laptop on which the Linux installation won't freeze after launching x11. On the one I have currently, no distribution could be installed. I believe I have to add or remove something to x11.conf but there is very few information about it. In the past I have used Red Hat and CCRMA at 2.6 ms latency (Shuttle XPC) without any problem.
I have a very cheap laptop: HP nx6110, celeron M, GMA900. no problems at all, FC5 recognized everything and with a little effort I also have a dualhead 3D accelerated setup (so I can patch while GEM windows is running fullscreen). also ubuntu runs just fine on it. I suggest looking here before buying: http://www.linux-on-laptops.com/
Be aware that is still somehow experimental.
Does it means, that I may get sound drops and variable reliability ? I need a simple setup, but as professional as my former Yamaha hardware sampler (1995) :) This machine had a polyphony of 64 btw, which is quite hard to get on a laptop (considering that latency has to be inferior to 5 ms)
It means that jackd sometimes crashes when I do strange and CPU intensive things, for example I am working on a flext external that does a lot of things in a different thread: sometimes when I run the separate thread jackd aborts.
Normally, once you have correctly configured jackd and the firewire port priority, you don't have xrun.
I don't think you'll have problems using it as a sampler, nontheless you might want to wait freebob to be "stable" and included in the official jackd distributions before using it on stage..