The best thing to do is get a killawatt http://www.p3international.com/products/p4400.html and check how much your idle system uses. Then lookup the maximum amperage used by your cards and get something good enough. I try to stay above the maximum so theres some room.
So, I've been a long time bitcoiner, but don't post at all. I've actually received my Jalepeno and the advice I can give is to not buy from BFL right now. A fellow friend of mine purchased his Erupters less than a month ago and has already received them. In terms of ROI, you need to do your cost calculation on whether it is worth it according to your power costs, initial investment and whether your erupters can turn over a profit at the current difficulty levels.