As i know, most of these chat softwares don't support keyword match. As funaki said, you can use google analytics to track keyword match. One of chat software i know is 123flashchat.com, it's good and fast, try it.
Rachelle, try Sales-n-Stats software, it's rather good, allows to chat with visitors , also generates different reports. Also you can see what search phrases were used to come to your site, that's really important to my mind.
I want to know your opinion. Does it seem to be irritating you when you browse on the site/store and suddenly operator invites you to chat with him? Or not at all?
Based on our experience, Liveperson is the best live support to monitor your traffic. Its integrated feature and applications like live chat, live call, e-mail support, and knowledgebase is worth to watch out for.
If you are after the statistics of your traffic, you should try out Google analytics as other have pointed out.. Your Cpanel should also have the feature to track down the keywords that bring in traffic to your website