I replaced the head unit with a Clarion NZ501. It's a single DIN unit that has a retracting screen that includes GPS and Bluetooth integrated. Two available optional components provide HD radio and Sirius/XM, for those interested.
The OEM crap Infinity amp that was located under the rear seat, was replaced with a cheap Alpine 4x75 amp I got on eBay and the speakers were replaced with Alpine R series 5 1/4. The fronts are component units (car came with separate tweeters in the door front corners). The rear speakers are two way - tweeter inside - units.
Install was very easy, sound is absolutely great and the bonus of GPS and BT makes a big difference...