depends on the motherboard, some mother boards have two slots, one that runs at 16x and the other at 8x/4x,usually the first one from top to bottom is the 16x one which is better, other motherboards have both kicking at 16x, in which case it wouldn't really matter, i would still use the top one though just for aestheticx, even that evga motherboard that has 3 pcie slots only has 2 that go at pcie 2.0 16x and then the other one that goes at regular 16x, if you only have one card then its better to put it on the 2.0 slots to get as much bandwith from the video ram, if your card supports those speeds of course

as for the 2 cards better then one, i guess that would depend on the actual specs of the card in question, ram speed, clock speed, bit rated specifics would probably determine the difference of the card, and how much you can overclock it if you plan to, lets say you get two cards that each have low clock speeds but decent ram, you could probably get more overclocked speed from the two cards then you would the single card if you consider speed per unit of heat, if you were going for a two card setup i would go with crossfire just because of the more scalable options that you would have at cross-referenceing the cards during use then sli, but if you had money for 3 cards id go with the sli, although technically if your willing to modify that single card and there wasn't a horrible bottleneck when overclocking the gpu, you could change out or install if it doesnt already have heat spreaders on the chips of the cards and then maybe change out the cooling fan on the card to something with high rpm, you could probably outmatch the sli cards with a single one, although the life expecttency would probably be the biggest factor to juggle on when going that route. if your not going to overclock though, then i would go with a nice big one and save up and get another nice big one later.