Fortunately, I’ve landed a few jobs recently involving heavy work in After Effects. While for the most part the work’s been extremely fun, trying to get fast/full RAM previews was really frustrating. I often found myself purging all memory every couple of minutes. To top it off, after a recent upgrade to CS6, After Effects began to display the dreaded ‘RAM Preview needs 2 or more frames to playback’ before EVERY RAM preview. After a bit of research and experimentation, I’ve found for my Macbook Pro (2.7 GHz Core i7, 4GB RAM) that a disk cache of ~10GB seems to be just about right (although this will obviously depend on the type of work you’re doing). Turning ‘Render Multiple Frames Simultaneously’ on had very little impact. Multiple forums seem to agree, claiming that more RAM would be needed for that feature to be beneficial.
Although I no longer really need to, I will often purge all memory (both from ‘Edit > Purge’ and using the ‘purge’ command in Terminal). I’d say it would probably be more important to monitor the disk cache however (and empty its contents if it’s close to your limit), as it will sometimes fill up without releasing.
Since doing this, I’ve never seen the error message, and RAM previews and final renders are a LOT faster and longer than before. I’d also recommend upgrading to a SSD… the benefits of that alone are insane.