I would have used the money to get at least a 15" with a dedicated gfx card.
At the low end you're looking at $800
Not worth it if you ask me. The 13" uses 256 mb of your ram. No more. More ram would only assist with running programs and what not, I doubt it would impact the video much if at all.
I would spend the money on a nice SSD and get a real performance boost.