Subs and door speakers
 

You might want to see what's available on Crutchfield. They have a ton of free info.

Loud and good are relative terms but for $500 you can have a huge improvement over stock. I grabbed a JVC head unit with bluetooth for $80 and two sets of 2-way Fosgate 5 1/4 for $100. Sounded decent (for a weekend fun vehicle) but lacked any bass. I had an extra amp, sub, and wiring kit laying around but that can all be had for a few hundred if you find deals. Boxes for a single sub are cheap or you can make your own and save a couple bucks. +1 on Crutchfield, been using them as long as I can remember.
Loud? Yeah, it'll annoy anyone next to me at a red light.
Good? It wont win any trophies and any true audiophile will scoff at it but it's more gooder than the Walmart radio and 3 blown speakers that I started with.

Head Prices
 

I can't believe how cheap these new head units are ?

Ebay and Craigslist are great for used equipment

Head Unit - two options here. Either a full system, or get an all-in-one amp/headunit combo
- Look for a Kenwood KDC X-701 or 702 because it has;
APT-X Bluetooth (if you have an android), Multi-phone connection, HD Radio tuner, 5v pre-outs. I got mine for $60 on eBay open-box with all accessories. Only one scuff on the faceplate.

- Then buy any 4-channel amplifier with 45+ Watts per channel. I would buy this used also. I bought a JL 4-channel amplifier for about $90 on eBay but you could go cheaper.

OR just get one of these, it has a 45w amp built in. Just run a power wire to the battery and find a solid ground location for the radio.

- Speakers are personal preference. With 45+w per channel, most speakers will sound good. I like silk-dome tweeters because they aren't harsh at high volumes. I went with Infiniti COAXIAL speakers, but you 100% should get COMPONENT speakers instead. I recommend the JBL GTO609C

- Then figure out what kind of subwoofer you want (Ported vs. Sealed), and where you want to mount it. I went with a SubThump enclosure in the "cubby" area in the trunk. I think this was around $120 shipped.

- Subwoofer & amp, again - depending on your needs. I went with a SoundQubed 8" 600w RMS subwoofer with a Pioneer 600w RMS amplifier. $100 for the sub and $40 for the amp off Craigslist. Had a 12" ported Apline Type-R on 1000w RMS and it unfortunately was stolen. Went with the SubThump cubby enclosure to reclaim some space and keep things more stealth.

- RCA Wires, and Power + Ground wires, I went with KnuKonceptz but you can go cheaper. I recommend running power down one side of the vehicle, and RCA's down the other side. Also don't forget the radio wiring harness and mounting adapter bezel. Get one without the CD pocket, they just collect dust and it doesn't look as clean

- Sound Deadening - most important mod. I went with Noico 80mil thick deadening. Do the doors, rear quarter panels, and trunk for sure. The floor you can use something cheaper like FrostKing duct insulation you can find at home depot.

I definitely spent more than $500 total, but I did it in stages so it wasn't that bad.

Important things to remember:
Ground the radio and any amplifiers to the same location. I usually run a ground wire from the head-unit back to the amp. This will help minimize any grounding loops and potential between the two components.
Do NOT unplug or touch any of the RCA's when the head unit is powered up. Any feedback and grounding-out can blow the pico fuses in the head unit. Do not touch any wiring while the head-unit is powered on

Both of my amps fit under the rear bench seat very easily and neither has had any heat issues, even when driving 2000+ miles with the stereo cranked to 11.

Make damn sure that you wrap any loose wires and ANYTHING that can rattle freely. Get a new set of door mounting clips too because at least a few will break when removing the door panel.