I do not know if you should change from Fring to Skype, if you are using Fring to chat with your pals and not really into voice talk then stay on Fring, IM+ or whatever you are using. If you really want to improve the sound quality of your calls you should move to the Skype app.
The rumor is that the Skype for iPhone allows you to make VoIP over the 3G network on the actual iPhone 3.0, but you have to keep in mind that the 3.0 is a beta, and probably they will lock this feature for the commercial version of 3.0, but that is something that ATT will decide, after all... they will loose a lot of minutes if you are able to use the Skype over 3G.
The good thing when Skype runs on 3.0 is that you will have background notifications, to know when someone wants to contact you, because now you need to have the app active in order to communicate... not very cool...