Thought I'd update this, in case someone else had a few questions...
Just got done with using Skype pretty much daily, calling home from KL and Singapore. Used:
>Pocket PC with a wi-fi connection
>Mid-range laptop
>Hulking dual-processor workstation
Latest version for Pocket PC, v 2.0~, and v2.0~ for Windows. Headsets except for Pocket PC. All was being sent to our home office, which has a high speed (1.5) SDSL connection.
In between the Dell Latitude 620 and the 2-month old workstation, there was no difference in performance, sound or video. So my take is, that unless you're using a P2 processor or suitabally ancient, there should be little performance difference (if any).
I used two different connections, one hotel had a T1, the other was ISDN (both shared). There was definitely a difference. The T1 has clean and stutter-free, even with video. With ISDN, I was forced to turn off the outbound video to maintain good sound for my wife, and that would even hicup both ways once in a while.
Still, despite the hicups, as long as we were computer-to-computer, the connection was great! Almost no lag, great sound. With the T1 connection at the JW Marriott in KL, the video was good, too. The sound was better than my cell phone when calling home.
Using Skype Out was good, but not great. Obviously the computer that makes the connection for you at the other end is virtualized, because there is a big drop for the worse in sound quality. Not quite as good as my cell phone, either over T1 or ISDN.
My Pocket PC/cell phone, a Qtek S200 (HTC Prophet) just barely has enough horsepower to run the program. It worked, but with lots of stutters and dropouts. This was over Wi-Fi (g) to the T1 connection. I'm sure this was because the S200 has a 200mHz OMAP processor and is kind of weak. My wife has my old PDA2K, and when she called me (me laptop, she PDA2K) ift sounded like the Skype Out connection. So the PDA2K's Intel 400mHz processor is enough for the job.
Conclusion: As long as you have a reasonable computer, the big limitation is bandwidth. ISDN speed is barely enough for good quality.
If you discount the setup time for the laptop, Skype was great for us. We'll be using it from now on; computer-to-computer it outperformed cell phones and land line, at a price that can't be beat! And as soon as the next generation HTC phone is released with a speedy processor, I won't even need to deal with the setup time.
All the best, James