like for example if I get a computer with a GF2 card, compared to a ATI 9600 card...will the image between the two computers actually be notically different or just performance?
in games the image will be much different. I have a GF2 and my friend has a FX5600. When i play halo on my computer, i dont see the actual grass blades, i just see a green blob. He sees the true detail of the ground/warthog/guns. I don't
Barring any major difference in build-quality of the cards (didn't a lot of GF2s have slighly fuzzy output?) 2D images shouldn't be affected. There might be some minor differences in video, due to slightly different hardware implementations of hardware accelerated playback & resizing but I doubt you'd ever notice unless you're really looking for them.
3D, you're going to see a world of difference once you bump up detail levels and AA/AF settings.
From my experience, I can't tell with just pictures. But gaming is a whole different story.
I upgraded my Nvidia Quadro Pro 2 to a Radeon 9800 pro and I see no differences in 2d stuff like photoshop and looking at jpegs/gifs/bmp/etc. But with gaming, the performance is faster and image quality is better with 3d stuff.