I don't know what the lowest spec amongst those who have sung/ voce chatted but people have managed to connect and listen/ text chat with as little as a 486. As Ed has said, the chances are that you will be ok - and the way to find out is to try it.

I used a P133 for ages before upgrading to a 500Mhz PC and I have noticed no difference in performance with Hearme although it has speeded a lot of other tasks up considerably.

Jon