I dont know exactly how many watts the oem system puts out, doesnt really matter any way. The answer to your question has a lot more to do with what else you are planning on doing. In the doors you have a 9 inch +/- woofer and a small tweeter (1.5 or so). The signal goes through a cross over before it gets to either the tweeter or woofer, so hooking up aftermarket speaker and getting them to sound good will be a challenge.
On the other hand if you are looking to replace the speakers and amplifiers, then you should be able to find a way to get pretty good sound. Going this route will be $$$$$ though.
Basically I would not recommend going with aftermarket speakers unless you are prepared to dump a lot of time and even more money into the project.
The stock system is okay. Ive heard better and worse. I have put really good stereos into cars in the past, but just dont think it is worth the cost anymore. It would take a lot of money and install time to improve the sound. At most I would use the signal going to the rear speakers to hook up a small sub. Anything more than that would be a waste of time and money.
If you want a great sound system in your 8, get a high quality mid-pipe and aftermarket exhaust combo. You will not regret it, and you wont need your stereo anymore for entertainment.
