A few points to make about live chat.
If you offer it, then make sure you have it "live" most of the day. It's a real negative to offer the service, and display the "offline" image for most of the day. If you're going to offer it, offer it.
A real turn off for me is seeing a popup window inviting me to chat. These windows are usually timed to appear after a set number of seconds after you hit the site. Personally, I find them intrusive and annoying.
You need to make it clear as to the purpose of your live chat. Is it for sales? For support? For both? I can imagine that it would be a real nightmare if you had some server/connectivity issue, and then suddenly dozens ( or hundreds/thousands) of customers hit your live chat to find out what's going on with their site. Logistically, there would be no way for you to handle the queries via live chat.
Vito