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.
dufrain
Thanks again, I appreciate all the help. Looks like I've got a decision to make, but you've made it much easier.
A little more information to help you in your decision.

My previous receiver was an Onkyo TX-SR606 that was rated for 90 wpc and continuous 6 ohms performance. My new receiver is an Integra 50.1 that is rated for 135 wpc and continuous 4 ohm performance. The improvement in sound quality was dramatic. I don't know how much to attribute to Integra vs. Onkyo, 135 vs 90 wpc or 6 vs 4 ohms, but the spec differences are similar between the 708 and 808 that you are comparing. The difference is that both are 4 ohm receivers.
Unless your HT room is 10'x10' or smaller I'd return the 50" plasma and pony up the extra $300 for the 60" and deal with the audio after the holidays. Everyone will give you krap about, "is your TV big enough". The second you turn it on all you'll hear is wow.

Does LG stand for Lucky Goldstar?