Cart before the horse or horse before the cart?

So, I'm finally building my first home theater and just purchased the TV, (LG PK950)so far very happy with my purchase.

My question is, if you were going to be starting with nothing, would you buy your receiver first based on your needs and then the speakers, or would you buy the speakers first and buy your receiver that best compliments your speakers? I've read so much about different receivers better suited for different speakers.

I'm undecided on speakers at this point, but I know I'm going to be buying used, betting I can get better quality for my money.

Paradigm Studio 20's or 40's
PSB GB1
Usher S-520
Energy RC10
B&W 6 series, etc.

Reciever: Not sure at this point, Onkyo, NAD, Marantz, Denon...

Honestly, there are so many opinions and when I think I've got it figured out something else comes up...need a little help please.

Budget:
speakers $500-$600 (used bookshelves)
receiver $700 max

For now, just the two bookshelves and receiver.

System dedicated to mostly HT and some music, IPOD and vinyl, punk and garage.

What do I purchase first? Thanks everyone.