how to minimize server load on ubuntu 12.04LTS

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http://serverfault.com – I am using ubuntu 12.04LTS server alongwith LAMP module. I configured my server with 2GB memory but after some time either i have to restart my apache2 web server and mysql process or think about upgrade memory as there is lot of database write operations on my server near about 70k per hour. Just for benchmarking i stopped mysql and test application without database server still heavy and doesn't working properly. On using TOP and free -m, memory resources still shows occuppied like out of 2gb only 100mb free. In this case what to do, should i go for upgrade or minimize incoming traffic reque (HowTos)