I will be doing another custom build in the next fortnight and I was originally going to get 8gb of RAM which I know is plenty for just gaming, but I read somewhere that photo or video editing can be much more demanding in the RAM area.
For photo\video editing and 3D rendering is 8gb enough or is better to bump it up to 16gb, the price difference doesnt bother, but I want spend extra money if its pointless.
I don't know. You much do you value speed? If you're really crazy about speed, you could use a superfast SSD coupled with some superfast RAM as RAMdisk. You're probably going to want a Corsair 240GB Neutron GTX or equivalent paired with 16GB of DDR3 2000 RAM.
For gaming 8GB of RAM is more than sufficient.
You are correct that video editing (Photo editing not so much unless your talking big resolutions) is more RAM intensive, so having more of it will help. 16GB is the usual amount for editing machines, though if it was a proper workstation rig, 32GB isn't that uncommon.
Though the CPU is a bigger decider in rendering performance than RAM is, pairing 32GB of RAM with a Core i3 will still be slow.
When I render video's on my 1100T x6 4.12ghz, My CPU usage in at 100% on all 6 cores, and my 8gb ram is is being pushed to its max. So if your video editing and or rending, I would get the 16gb kit or better. 8gb is more then enough for gaming tho, heck even 6gb is enough for gaming right now, but you can never go wrong with having to much ram, it wouldn't slow down anything, at the most it speeds things up video editing wise.