if your already on the lowest speaker impedance for max power it will only double it as you will have to double the speaker impedance as you've doubled the voltage but can't pull any more current
But the TDA2009 is rated for 4R so bridging and driving 8R won't be a problem.
All good advice. The TDA series from ST Micro is all around what you're looking for, and the example circuits in the datasheets will get you rolling on a power amp section.
The best advice I can give is to get a few chips and start tinkering. I got sidetracked reading into the details for far too long before jumping in when I started tinkering with chip amps. It all comes together really quick once you start building them.
It depends on what power level he's after.
The power output from a TDA2003 will be a quarter of a bridged TDA2009 driving the same speaker or half if he goes down from 8R to 4R.
You'll only get 10W from the TDA2003 with a 2R load, a 14.4V power supply with it driven to clipping giving crappy 10% distortion.
With 12V, a 4R load and an acceptable distortion level, the usable power will be around 3W.
The maximum theoretical power output from a 12V bridged amplifier into a 4R load = V
2/(2R) = 12
2/(2*4) = 18W, for a non-bridged amplifier V needs to be halved. In practise the actual usable power will be much less due to losses in the driver transistors and the fact you don't want to drive it to distortion. As low power speakers with a lower impedance than 4R aren't widely available, the only way to get more power is to boost the supply voltage or parallel the speakers.
For 10W into 4R with low distortion and a 12V supply, you need the the TDA2005.
http://www.st.com/stonline/products/literature/ds/1451/tda2005.pdf