Get a sub first. Actually I would just stick with stereo until you have saved up some funds to get a
good sub.
If you were going to do it in phases, go 2 channel to 2.1, then 2.1 to 5.1 as funds allow: save up, get a good sub, then save up some more and get the surround and center speakers.
Just my opinion - its probably wrong because your speakers probably do ok with the bass output...