to sticks of 512mb will run quicker on a dual channel mobo than a single stick of 1gb. 3 memory slots, two slots same colour and 3rd slot different colour. install one stick in 3rd slot and other stick in slot one, usually the furthest away from slot 3.
check your user manual or post back make/model of mobo and someone will tell you the correct way to install if i'm wrong but thats how mine is configered asus A7N8X-E Deluxe.
What processor are you using. If its a Pentium which supports Hyperthreading, you should not basically see a big increase. I have 2 512mb running with a P4 3.4. It flys. I think 1GB is the standard these days, especially with all the new software coming on the market/new games.