More memory will (probably) not resolve your issue.
"Sometimes, it takes time to load UI & communicating with database."
Take a look at the queries that are used, in the UIs that load slow.
To increase the memory, you can use setDomainEnv and set the Xmx and Xms to a higher value or set (and use) the USER_MEM_ARGS parameter.
Some example are given here: http://middlewaremagic.com/weblogic/?p=6388
You can follow this link to get into weblogic tuning details :
Firstly you will have to identify the probable areas where the application is getting slow. These can be lack of memory/cpu, high i/o, network congestion or db response. Depending on the responses you can tune your overall application deployment.