12 Bit SPI DAC, with integrated amplifier?

I am working on a project with the Adafruit WaveShield and concept/begining code wise it's going well. The end project will be an "Open Source" lightsaber driver for making custom lightsabers. Fuctionally, it needs to read an accelerometer, play sounds and handle 3 PWM outs.

Physically... it needs to be small. 1" x 3" (Or less) is the goal. To that end I am thinking of making a "Nano-Shield" version of the WaveShield, using the Nano as the base "Arduino".

I have been looking at DACs and have found a handfull of ones that support some type of serial interface AND have an integrated amplifier (Usually Class D and 1.0 to 2.5 watts or so.)

I need some help figuring out what exactly to look for. If someone could help with some basic info and take the time to answer a few newb questions I'd appreciate it.

1) Does a 16 Bit DAC do only 16 bit or is it just "up to" 16 bit? (Waveshield is 12 bit I think?) Could I use a 16 or 24 bit for 12 bit audio? (Greater availability)

2) 2-wire, 3-wire, SPI, I2S, I2C, ohh my! I gather SPI is a brand name... are any of the above the same thing but the generic version? Is there any compatability amongst them? Any resources where I can learm more about them and why they are different?

I just ran in to a DAC that had the integrated amplifier and thought "Gee, what a spendid idea!" but it was also way overkill in all other ways. I guess I was hoping someone had the Ferrari engine in a Yugo DAC all ready to go.. LOL.

But, I have found all kinds of other acceptable Class D (And other varieties) of amplifiers, and I think I can use the SMD version of the same chip that the WaveShield uses. That way I can utilize the same drivers and software but save some space. (IE, this guy http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MCP4921T-E/MSCT-ND)

My hope was to find one that would work with some minor tweaking but i think the simplest is a seperate DAC and Amp and keep the circuit similar to the WaveShield. Use smaller parts, eliminate anything not 100% neccesary, etc.