Answer: As much as you can reasonably afford.<p>Reason: File system cache.<p>In modern Linux and Windows operating systems every file you read off the hard drive is stored in memory and only re-read after it has been write invalidated. This can reduce the amount of disk related seeks greatly in a mostly read based workload.<p>For your average home user between 6 and 8GB are pretty standard amount and will give good performance.<p>For power users and gamers between 16 and 32GB should be considered.
<a href="http://kmandla.wordpress.com/2006/12/25/ill-let-you-in-on-a-little-secret/" rel="nofollow">http://kmandla.wordpress.com/2006/12/25/ill-let-you-in-on-a-...</a><p>The Linux version (complete with sorting office analogy).