MapTools is a really cool virtual tabletop that can actually handle minis. There are different frameworks for different systems that can be downloaded to fit it. I've never tried it, but I know people who swear by it. I think it can also connect with Skype or a Ventrilo server or something akin to that.The only way that I've run an internet campaign has been PbP on RPGnet, which has been very fun but incredibly slow.
Oh man, MapTools is PERFECT. It's exactly what I was imagining. Thanks! It's almost TOO feature-rich... you could practically turn things into a video game.