In reply to:
But if the stock speakers sound ok to you, you can probably make a bigger difference for your money by adding a single 10" subwoofer.
Preach it brother !
I'm no expert, but after many installs, if your on a budget, I would suggest the following :
1. Amp up the stock speakers with a high pass filter to block the low frequencies which the stock speakers will struggle to produce. (most amps these days have this level of filtering built in)
2. Power up the single 10" sub to fill the low frequency hole caused by (1)
3. As you are in a a smallish car, I would recommend disconnecting, or fading way out, the rear speaker volume.
I would say this this is the cheapest / easiest way of improving your system, but others on the board will provide better advise if necessary.
I have run cheap brand but high power amps set to a low gain, and got life of 5+ years, so I would recommend this route personally. This would allow you to divert most of the cash to a decent make sub woofer, which is important.