8GB RAM CENTOS6.2 Server, memcached how much is the allocation reasonable?

There is a server, 8GB RAM installed CENTOS6.2 system,
Top a bit of memory information,
MySQL supports full-text search, accounting for about 1.5GB,
Each root (PHP) process 16-18MB,
Each Apache (httpd) process 18-60mb (mostly in 45MB)

17 Cron programs per second are required to run,
At the same time, with Google Analytics, the peak per second can reach up to 20page of traffic. (Daily 4.3W PG volume)

Now the server speed down, want to install memcached.
Ask seniors, like me, how many GB of memory can I allocate to memcached?
In addition, the main task of this server is through the MySQL full-text search (real-time information), memcached cache time set how much reasonable?

Thank you.

Reply to discussion (solution)

Introduction of caches, no meaning for real-time information

Introduction of caches, no meaning for real-time information
How does information at that time optimize speed? Nginx?

Introduction of caches, no meaning for real-time information
How does information at that time optimize speed? Nginx?

Multiple server CDN

Multiple server CDN
I have only one server and I do not intend to expand it recently. Wait until the day PG 10w+ time to consider again.

