8gb vs. 16gb ram

In general, once you have 'enough' memory for the programs you run, having more memory won't increase performance any further. However, multitasking will be quicker and smoother. Plus, loading the games themselves and game levels will be quicker. Less graphical anomalies in games as well. With more memory comes greater graphical detail levels in games too.

Why would more system memory affect graphics performance? Because graphics cards can reserve system memory for texture storage and such. My system's GTX 570 has 1280MB of its own, yet still reserves 2815MB of system memory, for a total of 4095MB of available graphics memory.

With your program load, I'd definitely recommend 12GB or 16GB. I bought 12GB for my Sandy Bridge system because I wanted to be a bit more future-proof than 8GB of memory could provide.