It depends on the mobo I think, it's best to just try both ways, and see what the POST says....mine has 2 one colour and 2 another, and mine are both next to each other...But I know a mate who puts one in one colour and the other in the other colour....Just try it and watch the post for anything about running in Dual Channel
it usually one in every other one, because the slots (from the socket to the edge of the mobo) usually go A1,A2,B1,B2, where the letter is the the channel, and the number is the socket number. you want one stick in each channel, in the same socket.
depends really on the ram, and what you do. If its decent quality RAM, and your doing some higher-end gamming, it should be relitivly noticable (maybe 5 frames per-second). If you just doing word processing, then odds are you won't see a difference.