I run 8gb and currently:
Total 8172
Cached 6351
Available 6180
Free 246
So its not using 1/32nd of the ram in my system. I would say go for 8gb, unless you have a specific application for more. Any more than 8gb for the foreseeable future (until at least DDR4 is mainstream anyway) is just a waste really! You will have upgraded your socket before 16gb is needed. Put the extra few quid toward something else. Even if itsa new gen of SSD, or a slightly better graphics card. Both will be noticed more!