I would say that it depends on the laptop.
A Mac with dual G4s? You should run great.
A Dell with a 1.3gHz Celeron? It'll puke and die.
Numerous people have trouble running the M-Box on PCs, but I haven't talked to anybody who had trouble with the M-Box on a Mac. That makes sense, because Pro Tools was written for Macs.
Here's what doesn't make sense; PCs tend to do better (fewer drop outs and lock ups) with Firewire interfaces, which were designed for Macs, and are rated as being slower than USB2.
By the way, Digidesign will be phasing out the
M-Box; they just came out with the
M-Box 2, which is $50 more... and $150 past your budget.
If your laptop is up to multitracking, and you want more flexibility than Pro Tools (which has hardware and software limitations), then think about interfaces like the
Presonus Firebox for $300, or the
M-Audio Firewire 410 for $300... and, of course, sequencing software.
NOTE: You can buy
Sonar Home Studio for about $100, and upgrade to their best software as your budget allows... for about the same total price as just buying their best software.
If you need to keep the cost of the interface below $200 (so you can buy sooftware), then check out the
M-Audio Firewire Audiophile or the
M-Audio USB MobilePre. Both will work with Mac or PC.
Considering the cost(s) of setting up a dependable computer with hardware and software, something like the
Fostex MR8 might be a good start. I haven't used one, but the features seem to be right in line with what you want... including a USB port for transfering digital files to your computer for CD burning.