OK, out of literally thousands of combinations, heres a start for you staying in your price range......
Audio Control LC6i 6 channel line out converter $115
CTX65CX Image Dynamics 6.5" 100 watt component set $160
BT8014 8" Bazooka Tube 150 watt Subwoofer $120
MRPF300 Alpine VPower 300 watt 4 channel amp (2 channels to components, 2 channels bridged to sub) $150
4 Gauge amp wiring kit Approx $50
That being said, It has been my experience that most people who want to upgrade because their factory system "isnt good enough" will start out purchasing inexpensive gear (inexpensive, NOT inferior), and shortly after will want to upgrade again, and again, and again. They end up spending thousands working their way up the ladder, and end up spending much more than they would have if they just jumped all in right from the get-go. Eventually, you end up with an awesome system, a shelf full of used stereo equipment, and an empty wallet.
If you feel like you may be one of those people, take some time, and do some research, learn all you can about car audio products, visit a local car stereo competition, and demo some vehicles. There are alot of us that are more than happy to demo for curious people. Most importantly, save up some more cash, and chose the components that are right for you, and your tastes.
Good luck in your quest!!
