beginner adc question

[pianoman]

I own a Presonus Firepod. I'm trying to evaluate whether better A/D conversion would be worth it.

My chain of concern at the moment is:
{AKG C414B-XLS or AT4050} --> DAV preamp --> Presonus Firepod (Firewire interface) --> Logic Pro on G4 733 MHz.
I'd care deeply about 2 channels and care about 4 channels at a time. The Firepod does have a S/PDIF input; I'm hoping that means I could use two channels (but you experts will tell me, I hope) of "good" A/D and feed it into the Firepod.

What's my best route forward? Some specifics: would I notice in a big way the difference between Mytec 96 and Mytec 192 units if I track 24/44? Would I notice a big difference between a "real" A/D converter and my Firepod?

BTW, from the RTFM department, I did some searches, but if someone could point me to a relevant thread...