Welcome to the forums, murukan.rpr.
Individual stock applications ("apps") running in the system can be cleared in the application manager or by installing an app from Market which specializes in clearing caches.
The application manager method is accomplished by clicking menu > settings > applications > manage applications > menu > sort by "all." Then, after the apps have all loaded on the list, click each one you've been using, such as browser, google nav, etc, and use the "clear cache" button.
The RAM in the system is cleared each time you reboot the device.