Guys I am thinking to have chat system in my forum. Before that, I really need to know wether it's going to be useful or not. I have also read it can make negative impact on new forums. Is that true ?
If not, suggest me which one to use ?
In most cases, yes, it will be bad for you. Instead of posting and developing conversations, your users will be chatting all day. But if you insist on having a shoutnox,
MyShoutbox is a pretty good one.
I installed ajax chat and run it from a link at the top of the forum. The only reason I have it is so we can discuss the building of the forum then I will probably remove it. It is awesome though since it is easy to install and it integrates with mybb. Has all the bells and whistles that you could ask for too.
Personally i wouldn't add a chat to a brand new site. Better wait till you got a good userbase and your forum can "survive" a little drop in posting.
I am not a big fan of shoutboxes either. They cause more load then they are worth, and is most likely something that you will have to remove if your forum starts to attract more visitors, but can't afford more expensive hosting...