I think the Tascam US 428 comes with a version of Cubasis VST, which could handle all your MIDI/audio recording needs.
When you talk about ProTools, I assume you are referring to ProTools Free, since any other version of ProTools would require you to use Digidesign audio hardware, which you couldn't put in the Cube. I think that ProTools Free doesn't work with USB audio input, but I have read some reports of people having success using USB audio input devices with PTFree if they had Apple Audio Extension 1.3.6 installed. The problem is, this audio extension is only available on the OS 9.1 CD's that come with the new iBooks and TiBooks.
I use PTFree on my G4 450, using the audio input jack. To record audio, you just plug in and record a track. To record MIDI, you would do the same, except plugging into a MIDI jack (I'm using a Midiman USB Midisport 2x2). However, if you want to have a song to put on a CD or something, you would have to record the audio from your MIDI sound module while you play back the MIDI track. After you're all done mixing and recording, I think you can export or bounce the song to the HD; I'm guessing it outputs to AIFF, which you could then burn to a CD.