change the respective pref settings. Otherwise you will have to provide proper system info, which you haven't done yet.
I've got a Mac OSX, 2.5 GHx Intel Core i7 with 16 GB 1600 MHz DDR3, which should be enough memory. But I keep getting the warning.
That's drive space not ram. Open up preferences and look for Memory and Disk Cache. Reduce the cache size.