Trying to understand tub/shower plumbing
	
	
		Good afternoon, 
I am having a contractor come in to do some renovations on my house, and one of the things he is doing is replacing the dated tub with a four piece unit.  He said that I needed to provide a shower valve, and of course we'll need to get new fixtures as well.   The set up we have now has three knobs, one each for hot and cold water, and one to divert the water from the tub faucet to the shower head.  
My question is this - I know this is kind of a dated set up, as evidenced by the fact that the big box stores don't seem to have much that fits it.  When the old tub is ripped out, can the set up be changed?  I guess I'd like to set it up like most showers are now, with the pull bar on the tub faucet that diverts the water.  
Also, I'm not entirely clear on the purpose of the shower valve.  I know they regulate water pressure, but is that all they do?
Thanks for any help you can provide.  I'm a total novice at plumbing, and I don't want to go out and buy the wrong thing!
:o