Then you should buy more ram or reduce the size of your document. Do you suspect that you have unused ram? What is your operating system?
i am using Vista and photoshop says i have 1654 MB of available ram.
You did not say which verson of PS you are using. If CS5 and a 32 bit system you are handicapping youself.
Works best in 64 bit mode with 6-8 gigs of RAM.
If you are limited you keeping what you should also check you have enough free area on the scratch disk.