$600. I can't decide how to split that for you. How big is your room? (volume and is it open?). Then, what is the system used for? Like... 80%HT 10%games 10% music? I will say one thing, once you have "solid" bass in your system.... you will be spoiled rotten. You will never go back. 50/50 split on sub/speakers is not a bad idea, but I personally would get the best sub possible, and eek out the fronts, used or whatever. (assuming HT primarily. my system is also for 2-ch and my mains cost as much as all the other audio equipment combined).
Yes, a better sub will help your overall system, and how it blends. But, then again, all of it matters. Placement of sub, and placement of listener position are paramount. (Living rooms are guaranteed to provide plenty of issues with humps/nulls).
Don't worry about the rear speakers. In fact, erase them from your mind for now. They will come naturally. Don't even bother to even think about them, till you get the sub and/or fronts.
Well, I think you usually get more speaker by getting floorstandings than bookshelves+stands at the entry level price point. (Obvious reasons on both counts). But your response makes me wonder about even just placement for bookshelves. Yes, they are called "bookshelves", but actual bookshelf placement is normally quite a compromise. All speakers like some space on all sides.
OK, so in a nutshell, I like floorstanding. But, what I am saying is that no matter what you get, think about good placement (as I already touched upon with the center channel). Hope this helps.
If you want more help, better explanation, diagrams, pictures of your setup and room might be in order....