Power-combining monster-cluster?

Is there a way to combine the power of multiple (completely different) computers by networking them together somehow to make them act as one powerful computer? Example:

Computer 1 = 1.3 GHz with 1 GB of RAM
Computer 2 = 1.7 GHz with 2 GB of RAM

This should result in a computer that acts either as:

Result 1 = 3 GHz with 3 GB of RAM
Result 2 = dual-core 1.5 GHz with 3 GB of RAM

The solution should be scalable to several computers. It would be preferable to end up with it running Windows or some Linux distribution (possibly with software-based virtualization).

I've looked at clustering software/tutorials, but everything I read seems to contradict everything else I've read. If I'm getting the gist of it, the above is possible. I'm wondering (assuming that it is possible) how to do so.