Thank you very much Hendrick!
I had thought about firstly amplifying the signal - which has an amplitude swing of about 1.5 volts and bottoms out somewhere a little above zero.
And doing that - with an op amp to make it swing between 0-5 volts. And then perhaps follow that with an LM3915 which has 10 comparators with preset voltage increments. At least that would be fun to play with :-]

I wasn't certain if the PICs A/D would have enough resolution to follow a 1.5 volt swing and get the resolution down to 1/10 of a wave.

From what I've read - if I first try the A/D route - the formula I should be using is atan2(V2,V1)
But I've also read that some prefer to make the signal range from +5 to -5.
If that is case - I'm not familiar with a PIC having an A/D that can read a negative voltage.

I'll look into the S/H inputs that you spoke of, and the 18F2431 device family.
I'm not at all familiar with that - but I kind of think I can imagine why - in a sinusoidal signal - that might be applicable.

Any other hints you feel like passing my way would be greatly appreciated.
Thanks!