OK here's the deal, ATI/AMD, Nvidia all they make is the die, board and processor-Then passed out to the finnishers, XFX,Diamond,ASUS,MSI ect,ect. The quality of the new-born card depends on the parts that they install on it, capacitor's, choke's, resistors. thats where the price differace
comes in, follow me, kinda like a car bodyshop, you get what you pay for. lol.
And as far as the overall diff betrween ATI/Nvidia? they use differant type's of processor threading, I use to be a ATI person for a long time, (middle of the road, Im just not gonna spend over 250 buck's on a damn vid card) lol and not wanting to dis the ATI people, but for serious gaming/
need to study Nvidia a little more, (my opinion)