I prefer IRC with a dice rolling script. There is a thread on this page (or maybe 1 page back, 2 at the most) about IRC gaming, check it out for links to clients and other useful utilities.
I heard that AOL Instant Messenger has dice abilities built in (I know that the AOL internet service does, but I can't confirm about the instant messenger alone, because I don't have it installed) but if it is true, gaming over AIM might be good, especially since it has voice-chat and even webcam support.
If you want to do it entirely by voice you could use a program like Skype which lets you have voice conferences PC-to-PC (up to 6 at a time I think?) and it is free, then you could also use a whiteboard program along with that for combats ... but
IMO there are no suitable whiteboard applications for gaming...
yet another option is something like OpenRPG, a suite of applications developes specifically for internet gaming. The downside is that there are a lot of things you must install to get it working, and not everyone likes using it anyway.
Overall you will probably need to try several methods and/or programs until you find something suitable for both you and your players
Let us know what you decide ... i'm always curious what others use