Upgrading RAM....really worth it?

Hi everyone, I have a general question regarding RAM. I am not much acquainted that much with the intense computer lingo but I may be able to understand the basics. I recently got a system with the following specs:

Core 2 Quad 2.66 GHz, 1333 MHz bus speed, 6Mb cache
Intel DG31PR Motherboard
2 Gb DDR-2 RAM
9500 GT 1Gb Accelerator

I don't do much gaming on PC as I have 360 for that, but I like to mess around a lot with Photoshop and video editing for fun purposes (not professional). I was wondering is getting another 2 Gb of RAM really worth the deal, or probably getting BOTH a new motherboard (as this one has only 2 slots for 2 DDR's) and then bigger RAMs in the range of probably 8 Gb?

I am always confused as to what "type of speed" does a C2Q processor offer and the type that RAM offers.

Now that depends upon your usability of system. Since you are more into photoshop and video editing only, I think 2 GB is enough for your system and for your usage.