for photoshop, and depending on how big the image/how much your editing the image, more ram will be better. if you constantly add layers/effects and work with huge image then you would most likely see some improvement with 2gigs. i dont think it's necessary but it wouldnt hurt.
for video editing 2gig will most likely help also. i dont video edit but it is alot more memory intensive then pictures.
there is a definite improvement from 512 to 1gig though for just about anything.
for videocard, most video card should be fine as long as it's not intergrated as that uses you system ram and is slower then dedicated video ram. anything around 60 should be more then enough but i recommend you spend a little more and get one with at least 64-128megs (dedicated and not TurboCached or HyperMemory) which will help when you are editing images.
you probably wont see huge differences, but you should feel the difference. like if you switch between applications or refresh images etc will be noticable faster, not alot for everyday usage but with lots of stuff on screen it will be.
also make sure you know what type of free slot you have, if it's agp or pci-express before you buy as they arent compatible.
so if you have a choice between 512->1gig and dedicated video card, you'll probably see more difference with the ram increase.