I'll second Boner's notion that MSN is (just barely) the best option. Good quality, not too much dropout and delay. The only annoyance is that it requires MSN Messenger (*blech*) and it might take a few tries to get the connection established. Once you got it going though, it's pretty damn good.
Skype isn't bad, quality isn't as good as MSN, but you don't have to deal with MSN Messenger. My girlfriend and I use Skype to talk as we play Neverwinter Nights together
Its a small app, so we don't notice any more system resources being used, and it doesn't appear to create any lag for the game either. We have noticed a bit of lagginess/choppiness with MSN though.
I've also had some recent experience with Dialpad, and didn't really like it. I experienced significant lag, which got really annoying. I was making a PC-to-Phone call between Europe and the US, so distance might have something to do with it, but it sill sucked.
As a side question: does anyone know of a thrid party client that I could use for MSN that also has the voice feature? I use Trillian, but then switch to the MSN Messenger app when I want to use the voice chatting.