Under $800 eh....
I'll agree with Lone_Ranger on the front speakers and amp. Prolly like 65-70 Watts RMS. Get like a 150-175 watt X-plod amp or equivalent. For the rear i would suggest the audiobahn 105 Watt RMS speakers which are available at crutchfields or Sounddomain.
For the Head Unit I personally suggest a JVC Arsenal Mp3 HU. They are reasonable in price, very good quality for the JVC name brand, loaded with features, and pretty powerful.
For subs, since i've never had subs in a truck i'd have to suggest MTX Thunders. They have great reviews for depth, handling, construction, durability, and functionality. And i've heard from more than one person on this forum about how well they work.
I'd stick with an MTX Sub amp as well. Prolly like 400-500 watts RMS and you should still be under 800.
But the excess will prolly go to an adaptor for your new HU, new harness, and a wiring kit for the amp, unless you sweet talk a salesman into throwing one in.
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