Earlier someone asked if a fast computer could 'piggyback' for power processing - long story short, we could set that up but it's an evil little bitch and it's just not worth it. These guys' suggestions are probably the easiest way to go - get something cheap that will stand up to tracking needs, and then mix on a powerful beast of epic proportions.
The reason you don't want to slave out one computer and use it as 'reinforcement' for another is because it's very hard to make software optimize this and it would really just be quicker to use a laptop for mobile tracking and a desktop with guts to do the mixing. If you have a multi-core (like an Intel Core Duo or AMD X2) or well-done multi-processor (Beowulf cluster) system, what you're setting yourself up under is called SMP and requires that you have a workload 'split' between processors properly (which is nothing new if the software supports it, or a master's degree in computer science if the software doesn't), and anything that can be done is basically handled for you out of the box. If, however, you have two different computers working together, the optimization is an evil little bitch that's basically like trying to have an elementary school carry a Howitzer uphill in the rain - the little children can't do a whole lot, so in practicality they should just stay out of the way and let the bigger kids do a lot of the work until they're really needed, but you spend all sorts of time trying to figure out when the wee ones are helping enough to justify the chaos introduced by putting kindergarteners in the way of hundreds of kilos of steel and exploding things... basically the work taken to utilize the weaker computer's processing power is only worth it if the two computers are fairly similar in spec, but for that to be realized you're either spending a lot of money on both or crippling them both for budget reasons. Get a cheap laptop that'll do the job (decent speed, USB 2.0 or FireWire port, no gaping holes where screen should be) and splurge on a fancy desktop.
Jeff
