ok well im guessing your talking about the 350z...this is the wrong forum but thats ok. the 350z/G35 has its own forum under Z cars.
i dont know quite what your asking but i will try and pick through some of the question marks.
the 350z i doubt will ever come in Twin Turbo form any time soon. they are working on Twin Turboing the VQ for the upcoming GTR in 2007 so i dont think you will have to worry about that...
as for dealers overcharging, i guess they can charge what ever they think they can get for the car. im sure nissan doesnt care as long as they get "their" money out of the deal....so say nissan wants $18k from the dealer. the dealer sells it for $40k, as long as nissan gets there $18k im sure they will be happy!!!
as for the Z compaing with the RX8, the RX8 is already out and (im guessing) sells are doing well but nissan's 350z sales are enormous...Z offers more power, better handling, and better aftermarket.
looks for base model Z's to be just under $30k and the top model Z;s to be just under $35k...yea you can pick up better deals and what not but that is a round-a-bout!!!
if you have any specific questions just ask!!!