I know that Klooge has a config file done in XML that you can alter to fit your system's rules. It comes with a few different systems already done (D20, 2nd edition AD&D, d10, Hero, D6), so there examples of how to change it, and there is a yahoo group and a message board that has more information on modifying the file.

The others, I don't know as much about. Well, except that ScreenMonkey really sucked for us when we tried using it (it was slow and it crashed a lot).

OpenRPG is quite good. It uses XML for all the sheets and the like. I tried out Klooge and didn't much like it, and the hefty price tag is another reason my group and I are sticking to ORPG. I wouldn't have minded maybe $25 for the DM license, and $20 or $15 for player license, but the prices they are charging are out of the range that we'd pay (looks to be about $90 or $100 minimum).

Well, AIM an Yahoo are just chat boxes and I know you can create rooms in them just for your group. It's pretty basic stuff, but what else do you really need? The first adventure games on the computer were text.

Yahoo allows you to do voice. You could do that and use the AIM chat dice roller at the same time.

You could use Irony dice for secret rolls or create another room.