I almost missed that question
1) I use VoipBuster, and have manually set-up the Nokia client for the service. Works great, quality is superb, and I use it constantly, as I call Europe a lot. They have pretty decent pricing, and give you 270mins free a week in North America that is valid for 120days of last minumum recharge of 10 Euros. Since I talk a lot with Europe, that's not an issue for me. I've used them for almost 3 years now, never had any issues (I've seen people that had some, as there's virtually no support, but I've never had any at all, with billing or service). Of course, you could use virtually any SIP out there.
2) No, mine is a N95-2 (I think), 8GB EDGE only for NA. Got it from Plemix with Asian firmware (same as my E71).
3) The stock / bulit-in client in the phone (for both, the N95 and the E71). The quality of the built-in one is simply unsurpassed, better than GSM calls. I know some people use truphone, for example, but as far as I know it simply installs its settings and uses the built-in client. Fring would be different, but should use the internal codec, and in my experience, while much better quality than on any other phone I've had (WM), it is sub-par in comparison to using the built-in client.
I also use IM+ for Skype, since most of my family and friends in Europe use it. I have the ultd NA plan on it (I think I paid $20 or $25 for a year), so if they call me, IM+ for Skype re-routes it through the GSM network as regular call (and since the skype pro plan, this is free). I also use it ocassionaly for calls if have no wifi around and need to call internationally, or when I'm at work, it forwards the "calls back" my work phone (there are profiles for that).
P.S. If you, or anyone else for that matter, need a guide / walk through on how to set up the internal client with a voip sip service, either PM me, or I could post the "how-to" here. It is not the most straight forward thing the first time, but very easy to do.