From Menu, go to Settings>Connectivity>Settings>Destinations>Internet
Change your gprs to priority 1. Doing this now makes all your standard apps work on gprs.
Now to get Youtube and browser to use Wifi, open your browser and go to Settings>General>Access Point.
Set this to "Ask when needed". Each time you start the app, it will ask, and you can simply choose "Search for WLAN".
You can do the same with settings for Opera as you can with browser. Most apps that connect to internet have an option to set to choose how to connect. Only Ovi and Social I have noticed don't have any options, or maybe I'm wrong. Not sure about SSOUI, but after doing this it will connect with 3G. You can play around with settings yourself if you prefer to have it work differently.
You could also create a destination just for Wifi in the first part and move your wifi access points into this, then choose this destination as standard in the browser settings instead of ask when needed.
Under Settings>Connectivity>Settings>Data use in home country, you can also set this to "Always ask" then each time it connects to 3G it will ask if you want to do connect this time or always connect for that app. This just helps to prevent it connecting without you wanting it to if you select "connect this time".
Hope this helps.
Thank you very much! This helped a lot. I almost got it working/configured properly now!
I have put 3G to prio1, so Nokia Social, SSOUI, store etc. connect through 3G. Just how I want it.
Browsers connect through Wifi now:) Super.
But in Nokia Internet Radio I can't find the option to select accesspoint, so this connects through 3G now too. Maybe I can't find or maybe Nokia has not integrated this option. Maybe there is still some solution for this?
Otherwise, it works great!