I have a bit of experience putting stereos in GMT400s, and have to agree with most of what was said above. I have found that the Memphis 4x6 with the aimable tweets sound amazing in a lower cost system. They're a little deep, but they'll go in. Get a small 4x35-50 watt amp (RMS) and dont try to run the full wattage all the time and you'll have good headroom. I normally run the stock rear speakers because they are always turned down so far, but a set of cheap pioneers or sonys from Walmart will work fine too. I also recommend putting a set of good 6-6.5" speakers (I like Memphis because of the aimable tweets) in your front doors. This maintains the front stage, and fills in a little girth in the 60-140hz range-makes the snare snap real well. Run the dash and door speakers off of the amp, and run the rear speakers off of the head unit.