personally, went for 32, even though i don't 'need' it at the moment.
Previous pc was an i7920 with 6gb of ram. An old pc and 6gb was plenty when i bought it, but then games started asking for more and more ram. Nowadays, they seem to regularly recommend 16gb. So, looking ahead, and for £60 more i went for 32gb. Not an unreasonable amount so i don't have to worry about that in the future.