i say ati... since vista came out, nvidia has been slacking big time on drivers while ati were on the ball. when talking about the cards themselves, there are going to be ati cards better than nvidia cards and nvidia cards better than ati cards depending on how much money you want to put into it. however the "top card" award must go to the nvidia 8800gtx... i could careless because im not spending that much money and eventually something will top it.
crossfire can be done either inside or outside the case, and the cards do not have to be identical, only of the same series.
SLI must be dont inside the case and cards must be identical.