Receivers, amps & pre-amps?

[camper]

I'm no audio expert, and I'm planning (on-paper) a home theater project for the next couple years. Along with that is looking at equipment and trying to get an idea as to what is needed.

I'm confused with the relation between recievers, amps & pre-amps. I was originally planning on just a receiver (a really good one), but I notice a lot of amps & pre-amps being used in a lot of people's setups.

My questions are:

1 - Are they (amps & pre-amps) necessary?
2 - If so, at what point are they required?

I'm looking at using a large screen TV (62" minimum) instead of a projector, but I'm not ruling out a projector either. I want high quality sound even though I'm not expecting to spend 10k+ per speaker. To give an idea on budget, I'm looking at about 12-18k on equipment alone. While I'm of the opinion that spending less than that would be nice that budget isn't set in stone either....if spending 22k will give me an infinitely better system that will last 15-20 years (with maintenance) instead of the 8-10 year life of a 10k system I'll lean towards the higher end. Still, I don't want to spend 5k on an amp/pre-amp (in addition to the receiver) if I'm not going to hear any major difference on say...Polk LSi's.

I'm not interested in whole house audio either, by the way.

If anyone can give SOME guidance or give me a good place to go read to get the info that would be WAY awesome.

camper