Against forum rules to post a thread multiple times...
Anyhow, go to Smartphones, Cell Phones & Smart Phones at BlackBerry.com
- they have a comparisons page where you can compare the specifications of the 8100 and the 8520 (there is no 8500).
There are several threads on the differences between the 8520 and the 8900 (some consider the 8520 an entry level BlackBerry, with fewer capabilities than the 8900). Again, the comparisons page will tell you that.