it's great to increase your cards per rig; but i go for stability, and i personally prefer 3 or 4 cards per setup (slightly underclocked, undervolted), which is more manageable for me than trying to get 5 or 6 cards rock solid stability. i want to be able to walk away for weeks and not have to intervene.
regarding expense: if you are building open rigs 2x500w is cheaper than 1x1000w. for video cards, keep in mind before the november price increase 7950s were $200 and 280x were $300; See if you can buy direct or backorder from your supplyer rather than ebay or craigs. things will change quickly for 2014, so avoid paying for overpriced cards.