Thanks for all the replies.
I think I've settled on a PbP. The realities of getting people in a chat room at the same time were challenging enough without the learning curves of OpenRPG or MU*'s and though Klooge looks sweet, getting 4-6 people to fork out $15 each is kinda unrealistic. Rondak's might be great for some but ultimately I found the site cumbersome and annoying. Though I'd love to have real-time gaming and I have a definite soft-spot for dicebots (I don't know why, but for some reason I could sit in a chatroom for ages and do little more than roll dice...), it looks like PbP is the best, all-round, option for me.
Some questions for you PbP'ers.
How do you structure your games? Are there common methods like one post per combat round? What online resources do you use to compliment your games? I know of Irony games dice roller and mapping utility, but they're Java, are there any other solutions? How do you resolve combats and combat rounds and movement, etc?