It is not complicated, but it isn’t exactly intuitive, though apparently Snow Leopard changes the way this works so you don’t require a restart.
Now select “Energy Saver”.
Now select “Higher Performance”, it will ask you to restart for changes to take effect.
Once you’ve restarted, that is it, you’re using the more powerful GPU. My games gained a huge amount of FPS when I did this, and with the help of the Fan Control application, I was able to keep my laptop relatively cold.
To undo the changes, repeat but select “Better battery life” instead.
On another note, I notice the new Firefox takes into account left and right scrolling on the Mac now for moving forward and backward between webpages :).