> THere's a hidden problem with this approach. Unless the crystal clocks
> on the two soundcards (the gadget that generates the 44100 kHz clock
> signal) are EXACTLY at the same frequency, you will notice that the
> latency between the cards drifts. 

This is so true. 

> Check out Ensoniq Audio PCI. My TB Malibu works well, too. Both are
> available for under $100 and have very good specs for such cheap
> hardware.

I bought Ensoniq Audio PCI on net for $55.  Alsa drivers for Linux support
card with ONE full-duplex device.  Csound now supports Alsa drivers directly,
and having a single device means that you can do input + ouput from/to DAC
at same time.  Just a bit too much latency for realtime effects, but still
good for testing dsp scores!