I had the same crappy dash speaker only set up in my 2003 Boxster. I added and Alpine head unit along with an amp in the floor of the frunk along with some component speakers in the doors and upgraded alpine speakers in the dash. I didn't add any speaker boxes in the doors, just surface mounted the woofers to the interior panel, and added dynamat to the inside of the doors and the interior panel near the speakers. I'm not audiophile, but I think it sounds great now.
Here's a couple of pics of the doors with the 6.5 inch component speakers installed:
Here's a shot of the amp and crossovers in the frunk. I made a cover out of plywood with short legs that sits over the amp panel so I can still stack cargo in the frunk without it sitting on the amp and crossovers.
Head unit:
