Is there a big difference between the ATI 4850 and NVIDIA's 8800GS? Will use iMac for iPhoto, iMovie for home videos, ripping dvd's/mp4 encoding and will be the iTunes for 2 apple tv's. Had created another thread and no one could answer the above question. Posters keep commenting and giving opinions, but what exactly are the differences? Which would be better for what I need, considering what I'll be using the computer for?