Buying MacBook Air need advice?

I'm buying a macbook air (11") and don't know whether to upgrade to 1.6GHz processor (from 1.4) or to increase RAM to 4GB from 2GB. I mostly use my laptop for word processing and messing around online, watching videos, etc., nothing serious. Because I've had catastrophic computer crashes in the past, almost all my files are stored on an external hard drive, so I don't need a lot of space for permanent file storage. I'm totally computer illiterate so I have almost no idea what I need but I'm on a budget so I can't just upgrade everything. Suggestions? Does any of it even matter or make a difference for what I'm doing?