What is basic the effect of RAM on the performance on web-server running CentOS? As far as I know, if my applications are utilizing only 1.5 GB of RAM and I increase my server RAM to 4GB then also application will work with same speed because then also they will utilize only 1.5 GB. What are other effects on performance?
Having more RAM then your apps need means the OS can keep more data in the cache, which might help if you need the same files all the time (how much this caching really helps is very much dependent of the usage pattern of your apps, ranging from extreme helpful to irrelevant). If you do not, it won't effect performance at all or only very little.