For gaming, going from 6490m to 650m would be a huge jump, where you would probably see a 300-400% increase in performance.
Don't look at the amount of ram unless you are running high resolution 3D applications. For surfing the web they will do the same job.
A 512mb 650m would crush even a 3gb 6490m in gaming performance.
Is there any benchmarks for the new Nvidia yet??
I heard all these remarks towards VRAM isn't that important before, because with the right benchmark settings there really isn't much of a difference. BUT
Even in 2D every Windows needs VRAM, which means with loads of Windows open the animations get crappy and I can attest to that with my 256MB 330M. I wouldn't buy something below 1GB just for that annoyance.
Had Apple put in 2GB DDR3 RAM it would have shown better results.
Also in 3D res is not the only thing that matters different detail settings have a different memory impact. Many games by default don't let you enable certain settings if you don't have the necessary VRAM. 512MB is todays minimum. In 1-2 years that will be too little for any higher details.
AA settings increase VRAM use. With enough VRAM enabling 8x MSAA costs you 20% performance, with too little it cuts 40% of your frame rate.
Textures at highest level are the cheapest way for good quality with little performance impact if you can store enough in VRAM.
In my opinion most Testing in that regard is not thorough enough and VRAM does matter and if you want anything future proof, 512MB is too little and certainly for a fairly fast 650M. It should be 1GB minimum and given the cheap price and great performance a 2GB DDR3 version should have been the default in the low MBP config. 2GB GDDR5 in the high end.
Apple doesn't do it because they want the difference to be obvious to the most uninformed customer there is, as they have a lot of those. Also 2GB GDDR5 would have taken too much PCB space.
The last generation 560M by comparison was sold with either 1.5GB or 3 GB VRAM. 1.5GB was the minimum and that GPU is equally fast. 512MB on such a fast GPU is ridiculous. 1GB is good, 2GB wouldn't hurt either.
Good summary from another gaming forum:
If you want benchmark performance, the lesser RAM with greater clocks is better.
If you want visual quality, the greater RAM - space for more and/or higher resolution textures - is better.
On a low-power laptop GPU, the benchmark performance is going to suck anyway, so if you can get the one with more RAM without spending much, you should.
Note that my viewpoint is biased - I find Mass Effect 2, Oblivion+BC+OOO, and HL2CM to be much more fun to play than 3dmark.