I2S - how many outputs? / Floating point power


I am a software developer with some basic electronics knowledge,
starting to dabble in embedded dev.

For the raw processing power (especially floating point), I, at first,
now don't get mad at me *gg*, was interested in the pandaboard.
As it turns out, its I2S outputs are already used by that thing.

So, I found the current beagleboard xm.

I would not really like to port my floating point stuff to fixed
point, if the FP processing on the A8 is really so weak, but oh well.
Is integer processing (with the extra instructions fixed point stuff
takes for muls etc) much faster on the A8 than float ?

Now the core issue:

I am not experienced with digital interfaces and have just been told
by an electronics guy, what I would want to use for that one project
of mine, is I2S DACs since they support up to 192 kHz @ 24 bit stereo

That's neat, but I need a few more DAC channels than 2. At least 4
with really high speed like provided by I2S (by 4 I mean 4x mono, i.e.
2x stereo),
and then a few DAC channels extra that need not be so hi-res, e.g. ~
40 or even 20 kHz / 16bit should be enough, could maybe even less.
For this, I've been told, SPI or I2C DACs are available.

Now, I have not really found an overview of how many channels for
these interfaces the beagleboard xm supports, *simultaneously*. (and
I can really use freely).

Can somebody enlighten me on this?

And while we're at it:
How would these interfaces be accessed, software-wise, say if I'm
going to use one of the linuxes available for pandaboard?
Would I have to write a device-driver or something, or are libraries
for this stuff available?

Thanks in advance,
- unshaven