A friend of mine replaced his stock speakers in his old Saturn with some Boston Acoustics he got from Good Guys. No amp needed in this case, took 15 minutes to install, and sounded like 20,000 times better.
Keep the following in mind:
- You'll need a wiring harness to install that head unit. Cost is about $50 give or take. Coincidentally, installation of a head unit at most stores is about $50. I'd get it done there to save trouble. That's just me though.
- Stock wires SUCK. Period. Consider replacing those.
- Replacing stock speakers should be cake unless they're nonstandard or have funky mounting angles or depth. I've seen people have to reform the panels and stuff to get "normal" speakers to fit.
- Amp is nice, but not necessary if you're not a) blasting your music, b) a HUGE audiophile, c) running a sub. Aftermarket headunits put out a decent amount of watts/channel which should be adequate for most people.
- Consider component speakers (i.e. mids + tweeters + crossover). Sound separation and fidelity is much better but comes at a sligtly higher cost and complication of install.