I think the best way to future proof your notebook purchase is to spend as little as possible on it.
This usually only works if you're not planning to play hardcore 3D games on it, and are ok with an Intel integrated graphics solution.
For example, I recently got a 15.4" Acer Extensa 5620 for $700 at Best Buy. It comes with a Core 2 Duo T5550 (1.83 GHz), 3 GB RAM, 250 GB 5400 RPM HDD, which is quite enough to last a few years for moderate internet surfing/word typing/general use I think.
Then when the time to upgrade comes, it's a lot easier to get a new laptop rather than trying to upgrade a $2000 laptop with a new CPU, RAM and HDD. Just my opinion.