I installed Nimbuzz instead of the usual MSN Messenger simply because I use Skype and Gchat and some other messaging programs which come in one package. Check it out on Ovi.
With regards to your WiFi/3G question, depending on what you're using...
For EMAIL, go to: Menu > Email > Settings > *Choose your Email account (i.e. Gmail, etc.) > Mailbox Settings > Advanced mailb. settings (at the bottom) > > For both Incoming and Outgoing, select Access point in Use and choose your WiFi.
To add your WiFi network as an access point and change its priority (if you use multiple access points/WiFi networks) go to: Menu > Control Panel > Settings > Connection > Destinations > Select either Internet or Access Point (to add an access point). If you select Internet in the previous step, you can change its properties, such as the WLAN security settings, and choose whether to use this access point/WiFi "Automatically" (you'll see it at the bottom).
Hope this helped 
