A phone works almost like a PC, it also needs RAM to run. Think of the RAM on your phone as the RAM on a PC, the internal memory as the Hard drive and the Memory Card as your External Drive or USB Flash disk.
RAM and Internal Memory on a phone are two different things, so to free up RAM you don't delete music and videos it will not help. Rather concentrate on freeing up RAM. This is what you can do:
1. Try to reboot your phone
2. Try not to run more than one application at a time
3. Disable images in your browser
4. Install Mobile Guard it is free, it will show you how much RAM your device has, how much is being used and how much is free , it also allows you to disable some unneeded processes and apps that start up with your phone. You can also manage your internal memory. (There could be something better out there but that's what I use.)
5. Try to use a different browser like Opera and not the built in browser.
6. Scan your device for viruses.
If everything fails - Get a better Phone!