I would pick 6.5 because it is stable... 7.0 is too new and when it comes to OS it is not always a good idea to use the latest version! I suggest you to install the Minimal version and then manually install Apache, MySQL and so using "yum install..."
If you have 4GB of RAM or more then you should go with the 64-bits version.
Good luck and have fun configuring your new server!
I prefer to use Apache as base and then nginx over it as a reverse-proxy system for images only. I also prefer to install and configure the software manually so suit my needs and have complete control over what is installed. Haven't used Centminmod ever so I can't vouch for it...