ajax keep,HTTP Keep Alive benefits on AJAX site

How much traffic are you talking about? If you plan to use Keep-alive then you might want to ensure you have enough memory and lower the keep alive timeout to the lowest possible otherwise if you end up getting a lot of traffic it could hurt you. Sites with excessive traffic usually have keep alive disabled.

As for your AJAX calls, I would improve those whether you use keep-alive or not. I would cache the responses. For example after you run your mysql queries and generate your output, cache it to disk for a few hours (or longer if possible) and then on subsequent calls pull the data from disk if it has not expired. This will save a bunch and speed up things overall.

Also if you're concerned about speed, I would use image sprites for a lot of your image resources. I notice some of your images are placeholders and are 100% transparent, consider using css only for those. This will reduce your overall requests dramatically.

I would also enable mod_expires and add some Expires headers. For an example htaccess file using these and more good practice features look at:

https://github.com/h5bp/html5-boilerplate/blob/master/dist/.htaccess

EDIT

Jeepstone, I would recommend you don't enable keep-alive and maybe use a CDN and Parallelize your resources. You may also want to look at your database config. For example MySQL has low max connections and you might want to optimize slow queries, connection timeouts and ensure that you do not use any persistent connections.

Also if you're really concerned about the state of your web stack you can test it right now instead of finding out the problems down the line when there are a lot more real customers knocking. I'm talking about stress/load testing.