Everything is personal preference with everything.
Some people prefer SUV's and other people preferluxury sedans, even though they would both cost $50k.
If you want to know what I personally prefer, here's my logic:
You only need your mains to go down to the 50's. 40's if you want to spoil yourself. everything lower (and slightly higher) is the subs job.
That really rules out most floorstanders, except for small ones, and most bookshelves except for large ones. Since large bookshelves are cheaper than small floorstanders for no difference when using a sub, financial prudence insists on the large bookshelf. Plus your options for mounting are greater since you can use stands or shelving etc. and not just the floor.
The nice part is that for rears, you can use more matching bookshelves anyway, and of course the center.
LCR (left center right) speakers I think are the best for this. So my money now would go towards the best LCR's I can find.
That's now. Before it was buy the best floorstander you can afford since the sub will come later and you really want the extension. However, if you were to use the money to buy a bookshelf that you would for the 'best floorstander you can afford' you really are buying more speaker.
Now back to budget. You only have $600, so for a pair that's got alot of choices. You even could probably get a left, center and right but you couldn't get a left right and a sub.
Now if you wanted to get not such large bookshelves for the rest but large for the fronts, or just large all around, or small all around, you can get two systems:
For a lower budget -
Rocket/ACI ELT's. These come in LRS and CSE.
For a pinch higher -
Ascend Acoustics. These come in CBM 170 and 340.
For $600 you can get in the rocket/aci center and special edition bookshelves.
For $600 you can get in Ascend the 340c and the 170 bookshelves.
Or you can just get the big Rocket CSE pair, or Ascend 340 pair and no sub.
Or just get the basic bookshelves and an entry SVS or Hsu sub.
I personally prefer getting the entry sub and the big bookshelves and that should set you back a grand. But, it will be killer for music too, however bust your budget.
Seth
P.S If you want links:
http://www.**********/products_catego...akers&brand=13
http://www.audioc.com/elt/index.htm
http://www.ascendacoustics.com/