But what about gaming then? I'm a very light gamer myself. I have a PS3 and even then i haven't gamed for months. If i do game on the Mac it will be for non-graphically intensive RTS games. Anything else like FPS i will get for my PS3 as it's unlikely i will prefer a 15 inch screen. But i do want to know my options though.
To put it simply 512MB is currently enough on "normal" resolutions you will get good fps.
More VRAM primarily helps quality. The gaming engine will automatically adapt itself to the lower available VRAM and it will render frames usually almost the same speed. Those frames will tend to offer lower quality, more popping up textures and so on. In many games the available quality settings will be limited based on how much VRAM there is.
512MB is currently about to become the minimum any game expects. It is still being optimized for while the support for 256mb is being removed from games. They still run but very poorly.
More VRAM is especially useful in games with out door levels and great visibility. Flight Sim., BF3. CoD is more streaming and less VRAM dependent.
For speed (fps) alone it doesn't matter unless you drive high resolutions. High being at least full hd but it really hurts above that.
For quality more VRAM is always better and I would say 1GB should be minimum on a GPU as fast as the 650m. 2GB should be and is the most often used option with that GPU.
Apple is very skimpy with VRAM because they don't think it matters in normal OSX use, they don't care about gaming and it saves space on the logic board. 2GB GDDR5 are actually quite cheap.
I also think they try to make the low end version look deliberately unappealing. Otherwise they really should have put 2GB on the high end and 1GB at the low end.
The 650M replaced a GPU the 560M which was slightly slower and game standard with either 1536 or twice that in VRAM. There is enough processing power to render a detailed graphics but it can only use shaders if there is no data to work with. Nothing is cheaper than high def textures in speed vs. quality.