It's a slippery slope because once you put in better tweeters in the dash, the rest of the sound will really be lacking. Then you'll need to change the door speakers to get full range of sound. And finally you'll want a sub-box to add some bass.
As far as I know, aside from taking your car to a stereo shop to do the work for you, your only DIY options are to go with an OEM Audio kit which will start around $1K, or do similar to what me and a bunch of other guys have done(see my thread below)
http://www.ft86club.com/forums/showthread.php?t=58497
I'm very happy with my sound system now after spending around $200 bucks. (of course I got a deal on some parts)
I opted for keeping the OEM headunit as well. Good luck!