Speakers / Amp / Confused?

[bmnk]

Ok, So I'm about to wire up my 52' truck. I have:
"Secret Audio" from Custom Autosound

A small amp

Set of 3" speakers for the center of the dash

Set of 4" Polk Speakers with 45w RMS - 4ohms

Set of Alpine 5x7s with 75w RMS -4ohms.

The Amp power output is 50w RMS x 2 @ 4ohms
75w RMS x 2 @ 2ohms
150w RMS x 1 @ 4ohms

My plan is to hook the small 3" speakers to the deck -- Then hook the 4" and 5x7s up to the Amp in parallel.



If I'm thinking correctly the speakers hooked up to the amp will then run at 2ohms, and will be powered at 37.5w RMS.

My question is 1) am I right, or completely wrong?-- 2) would I be further ahead to only power the Alpine 5x7's with the amp?

Any help advice would be great.....