I would buy RAM from OWC or Newegg as mentioned (if I lived in the US).
Anyway, regarding video editing and the video card. In what I've read and experienced the jury is still out on how important a video card really is in video editing.
If you use h264, which is used in a lot of cameras, you will rely on your CPU to help you. h.264 is very heavy on the cpu, and requires a lot of horse power. Case in point: my Core2Quad with a 1gb 9800GTX+, is slower than my 13" i7. Both are equipped with 8gb of RAM.
It is true that with the new Final Cut Pro X and Motion that a lot of rendering tasks can be offloaded to the GPU, or that the GPU can help with the rendering. I am still uncertain of how much.
There is of course no doubt that the 15" is faster and better for editing, and that the graphics card is a plus. I am just unsure of how much. I've previously bought higher end Macs to get the better graphics cards and haven't really experienced that much of a boost.
But that's just me. I really like the 13", and love to not carry around my old 15"MBP and I just feel that it's a more beautiful machine. When I use it as a stationary machine I have it hooked up to my external monitor in clamshell mode. Now if you only have one computer and no extra monitor the 15" might be a better buy.
But I'd say go for the 13" i7 and get som extra RAM and a SSD drive.
EDIT:
It is true that After Effects and Premiere CAN use the GPU, but on Mac it's a very limited range of GFX cards.
http://www.adobe.com/products/aftereffects/extend.html
Neither the 6490M or the 6750M are supported.