Why are you assuming that the OP will have to replace his macbook in a couple of years?
It's just based off of my experience with other people replacing their MBAs within a couple of years due to them slowing down with OS upgrades and the demands of newer software. Many people that I talk to and see on a daily basis also end up replacing their MBAs or selling them in favor of other machines that offer more internal storage that still offer great portability for a better price.