Shaq Diesel

For those that have played around with car audio in the 350Z, I'm looking to finally upgrade speakers. I have a Pioneer Avic D3. I'm thinking of just installing speakers to begin with. Given the abundant amount of road noise this car gives me the privilege to listen to, how much of an improvement is it to run the speakers from a dedicated amp versus directly of the head unit? Also, how good are the stock speakers compared to a decent aftermarket pair?

Due to pending med bills from my accident I don't want to drop a ton of money into this right now. I am also in no hurry to go and remove the headunit to install all the wires for an amp right now.