All of these questions are in full swing in various current threads. Have you searched the message board for these answers? I have the time, so I'll go through it with you to the best of my collected knowledge...
-You can use all of your processors but it is advised to use a few less than you have so that your computer doesn't choke while trying to do other tasks. People have been reporting odd numbers for the best results (such as 3,5,7 processors etc)
-Protools can only access 3-4 gigs because it runs in 32bit. This is a major complaint right now, and we're all assuming Avid is hard at work to make a 64bit version asap. It will likely be a while as apparently it's a ton of work. Yes everyone else is already there-no they aren't faced with legacy hardware support (there will likely be some HD users that would be terribly upset if a stock version of PT was more efficient than their huge investment for HD)
-All that said, in order to use your system to it's full capacity, you can either wait for PT to "catch up", or use Vienna Ensemble, or other DAW's with rewire. I'm not entirely educated on this process but if you SEARCH THE DUC, you will find a vast amount of information regarding those methods.
-Yes, as far as I know that's the limit for ADC. I'm guessing you're asking cuz some Waves plug is generating a billion or so samples of delay?
I'm nowhere near as experienced as some of the members here, and if I've gotten something wrong here they'll kindly/quickly correct me, but by this time, they're likely sick of answering the same questions daily. Dude, you'll honestly find the best answers by spending a little time searching around here.