Apache to serve RESTful requests using PHP

Dear list, We're a developing a PHP-driven web service with a RESTful API, and we have a dedicated Linux server for that with 6GB of RAM. Since this service will be used by many clients in a concurrent manner, we'll face with a high-load on our web-server. But web-services are different from web pages, for instance they don't include images, or in this case we only serve JSON. I'm wondering what are the recommended configurations for the Apache web-server in these situations? Should we disable keep-avlie? What about other directives? Apache is our bottleneck, and we're trying to optimize it. Should we use nginx instead? Please let me know your suggestions. Thank you, -behzad