For the general question you asked, you should ask yourself whether it is more important to you to have deeper bass, or to have sound coming at you from all sides. That will answer the question of whether to get a subwoofer or surround speakers next. I personally would go with an SVS subwoofer first (the best one you can afford, as you want one good enough that you never need to replace it, though their bottom model is quite good), but you should consider the question of which matters most to you.
For the surrounds, when you do that, you might want the speakers on stands closer to the listening area, though whether that is feasible or not depends on things that you have not told us. And if space is really a problem, you might also consider in ceiling speakers for surround use, though I personally prefer freestanding speakers, so that they can be easily moved if one decides one does not like how they sound wherever one places them first.