You can certainly do it on AIM; I've been playing RPGA games on AIM for over 4 years now.
AIM's advantage is that it's easy to download / install, and easy to use.
The biggest drawback that AIM has is that it doesn't have any sort of integral mapping tool. If maps are important to you, you can use a numbered / lettered grid (like an Excel spreadsheet).
AIM does have a built-in dice-rolling utility. To roll dice, type into your chat room window the following command (including the //):
//roll-diceX-sidesY
...where X is the number of dice you want to roll, and Y is the sides. Thus, to roll 1d20:
//roll-dice1-sides20
...to roll 5d6:
//roll-dice5-sides6
I know a fair number of people who use and like OpenRPG (among the online Living Greyhawk community, it looks to have become more used that AIM over the past year or so). It, too, is free...but I've heard of quite a few people (especially those that aren't tremendously computer-savvy) who've had real problems getting OpenRPG to install or run; even OpenRPG's fans will admit it's not particularly intuitive. From everything I've read, the best way to get up and running on OpenRPG is to get someone who already knows how to do it walk you through it.
As far as advice on playing online, a few thoughts:
1) Accept that it's simply going to take longer than the same game would face-to-face. At a minimum, expect it to take at least 50% longer, and maybe more than that (esp. in combat-intensive modules). It's largely because most people type a lot more slowly than they talk.
2) When possible, prepare your "boxed text" in a Word document, or something similar, so you can cut-and-paste text into the chat room window -- it's a tremendous time-saver.
Hope this helps!
