The best way to find out how a normal person drives a car is to put a normal person in the drivers seat. Watching usertesting.com videos has shown me how cluttered and slow a normal person's computer can be. It also showed me the interesting ways those people interact with their clunkers.
Instead of having a second crippled computer for testing, I would rather use tools like Charles (http://www.charlesproxy.com/) to cripple my computer in a tweakable and consistent way.
This way I can develop, optimize for certain controlled conditions on the same machine, and verify my optimizations with real people.
Still not slow enough - you'll only be using the second computer for testing. In real life, not only will it be underpowered, it will be running lots of software stealing performance from your app. And don't forget the full disk virus scan at boot.
You can't combine it into a single computer, because even if it is only 80% of the latest and greatest, it is still way faster than grandmom, thus not representative of your customer base.