Apple will install RAM at any time - for a major fee (actually, I think they install it free, but they charge way too much for the RAM). Hard drives, Bluetooth, AirPort, etc. - these are all installable after the purchase.
However, given installation costs, it's often best to wait until you can afford what you want, and have it built to order.
If you know anyone even remotely computer-savvy, you'll save a ton of money ordering RAM from a reputable third party and installing it yourself (or, rather, via the acquaintance).