Please do not use 40MHz with 802.11n if using 2.4GHz and there are a substantial number of APs around you actively using channels within the frequency range you plan to use. Doing so may induce interference/increase unreliability for people around you. If unsure, please use 20MHz!
Reference material for my opinion/statement is here (read, do not skim): »www.smallnetbuilder.com/ ··· z-part-1
Here's a visual diagram showing exactly what you end up doing with 40MHz: »en.wikipedia.org/wiki/Fi ··· N-en.svg
If there are no surrounding APs using those frequency ranges, sure, 40MHz would be fine to use. But chances are (in this day and age) that's not the case.
Bottom line: if you care about speed, use a wired connection. No debate or arguments please. 802.11 -- all revisions -- is quite a disgusting mess.--
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.