Using wifi AND wired with crossover cable

I'm trying to set up a wired connection between 2 computers using a crossover cable (to raise the speed of data transfers). Both computers have wifi, and are on the same network.

My first attempt was a miserable failure - I followed instructions on how to set up the network connection, but it seemed to wreck the wifi connection. The only way I could get it back was a rollback!

What's the best way to go about this? I have wifi set to automatically assign IP addresses. Would it be better off manual all round? If so, how do I determine what figures to use?

Thanks for any help...