In most applications I think you will see VERY minor improvements with 240mm. If you are overclocking high and doing a lot of folding or whatever then you might want to step it up yeah. I used a 120 before my current 280.
Highly recommend making your own loop thou. Less restrictive flow, possible to fix, possible to clean and custom tubing shape, size, color. Very easy to do. Check out swiftech if you are in the USA or EK in jurope