WPA was a stopgap to replace the broken WEP encryption algorithm whilst the 802.11i standard was agreed upon. Once 802.11i was ratified it was marketed as WPA2. Most WPA-supporting network cards can be made to work with WPA2 with a firmware update, but there are some that are incompatible.
As for your IP address, the most important thing someone can do if they know it is attempt to attack your computer. If you're behind a properly configured firewall you're pretty much safe, but if the attacker can work out what software the firewall itself is based on, it may be possible for them to attack and compromise it, then open up TCP/UDP ports to allow them access to the machines behind. For extra safety you should use a hardware firewall/router so that it's separate from your computer. Software ones are okay, but more likely to be compromisable (a good example of that are the Symantec products which frequently get targetted by exploit code). Mac OSX is quite robust against attack though, so it's less of a worry than for Windows users.
