I've had a Tungsten C for years and love it. When I lost my first one, I bought a second. The prices have come down quite a bit. I haven't used it for VOIP mostly because there isn't a dominant application developed for it to handle that. I assume one will come out once a Treo with native WIFI support is released. It is adequate for surfing the web or IM and does a great job with email.

About the Treo 650... It does not officially support WIFI. However, a developer named Shadowmite on treocentral hacked it, and it now works. The thread is extremely long, so you may want to jump to the end. I found reading the entire thing interesting. It was a bit like a techno thriller novel.