I actually disagree a little with RPM. I think that most of the head units that manufacturers put in vehicles as stock is rubbish for the most part. I am unfamiliar with what they are putting in the SRT's, so I have to claim a little ignorance there. My friend has a Cherokee SRT8, and his is great. But for the majority of us who have a sub-$50,000 vehicle, they put poo in the dash to run our speakers, and crummy speakers at that.
In every vehicle that I have owned modded the audio system first. I would say that if you have the Pio D3, you should be set as far as head unit goes.
Next it is on to the speakers. Replace these with a solid set of 3 ways up front, and then some basic quality speakers in the back (If you even want to at this point, as I dont sit in the back seat to drive. Gotta look out for #1) You can look at your budget and get the best thing that you can afford. Dont skimp if you can help it. Look at what you ultimate goal with your stereo system is. If you want to compete in dB Drags, you will need the BEST you can get your hands on, but if you want to drown out your wife
you can just go with a good mid-range set of speakers. You wont necessarily need to amp them.
Reserve the amp for the sub woofers. Once you get to this point, you should have an idea about where your speakers stand. I would recommend going into a shop that you trust, and just asking what they recommend for you that they carry, and what they have in their cars. When I do this, I listen to the words that they use. If they say "These are LOUD!" and, "They are loud... Loud... Loud." I walk the other way. I dont want loud. I want sound quality, and Quality, and Quality.
Hope this helps.