Way it`s heading, 4GB(2x2g) is the way to go. 2x1g u can semi get away with, but tbh, with the Vista-factor, u waste half of it.
Outside of a `sold as` 4x1G quad kit, 4x1G works, but I (or the manufs) wouldn`t recommend it for people pushing their systems.
6GB will thrown ur timings off so much, it isn`t worth it - if ur an oc`er/get the best out of ur stuff - type person.