I was about to get a G20, the test drive was super-smooth. I settled on a slightly older Maxima but was happy with the extra power and room. I\'m sure you\'ll enjoy the car for years to come, Nissan makes rock solid engines.
For car audio go Alpine for the head unit, Rockford Fosgate for the amps, and speakers have a wide range of options.. depending on your budget. Infinity Kappa\'s are nice if you are low on cash, Focal or A/D/S if you really want serious sound.
Also check
here to see what speakers fit your car. I\'d put nothing less than 6.5" components up front, if your ambitious enough you can make your own baffles to increase the size restraints.