Gaming will run happily in 8GB.
Photoshop can use a heap of RAM, but it depends how large the images you work on are, and how many layers you use (and how many images you work on simultaneously). I put 12GB in my Photoshop box, but I've heard of people using 32GB (using Macs) or more. If you are working on reasonable sized images, 8GB should get you going. Keep an eye on the scratch drive - if Photoshop starts hitting the scratch disk heavily, it may be time to buy some more RAM.
3DS Max is something I have never used, but I get the feeling it's like Photoshop, with the amount of RAM it wants heavily dependent on the work you are doing.
Video rendering, depending on how/what you are doing, may depend a lot more on the CPU than on the RAM.
Anyway, if you are into constructing gigapixel panoramas, then editing them, then I suspect you really will want 16GB of RAM up front. Just be prepared to rip it out and replace it if/when 8GB DIMMs arrive. Don't worry too much, though; they could take longer to arrive than you'd expect - 4GB DIMMs took far longer than I wanted them to