Ridzuan, I haven't used cooledit to sample PCA data! However, you may need to use a resistor divider to get the voltage to the 1V input level of the sound card.

What sample rate? As fast as possible! Usually 44KHz - there's plenty of memory in a PC, so we can afford to waste a bit.

I usually use cooledit for diagnosing slow speed modem problems. Having virtually unlimited samples means I can capture minutes of data easily.

Frequency wise, it is pretty accurate, amplitude wise, not very, but you can calibrate it. Don't expect to get perfect square waves - the inputs are usually capacitively coupled so you get parallogram waves rather than square waves displayed. Great for decoding infrared remote control signals and audio band signals.