I want to buy four r9 280x cards for Scrypt mining. I already have all other hardware components to create two dual rigs. Would one mining rig that uses all four GPUs instead be significantly better?
I would have to buy a motherboard or riser cards to create the quad rig. I am indifferent to power consumption.
Theoretically you should see a net saving in electricity consumption for there ought to be some economies of scale in having a single motherboard with the 4 cards vs. 2 separate motherbs. each with 2 cards each.
The CPUs controlling such cards are consuming power themselves but such consumption doesn't get much bigger if you have 1, 2 or 4 cards in the same motherboard. Plus the inefficiencies in AC/DC conversion within the power supplies would double, so yes - you should definitely see some savings with that arrangement.
But then, such a machine would consume tons of power, which not a lot of power supplies would be able to yield - 500 Watts, 800 W? Plus all the heat they'd generate, which of course needs to be dissipated somehow - which means not just some big fans, but probably some heatsinks, Peltiers or just submerging the whole arrangement in a bath of mineral oil -- fluorinert would be even better, if you can afford it.
Such big initial investment costs could offset part of the savings, so I believe it would be barely more profitable than the 2 machines way.
Save your pennies and hold out for the upcoming scrypt ASICs. Companies such as alpha-t.net are planning to have product on the market by Q2 or Q3 next year, which will blow the pants off GPUs. Registration is open for their pre-sales, but they're sure to sell out of the first generation very quickly.
Your assertion about "throwing" is indeed correct... Mining with GPU technology has become a money loosing proposition with the advent of ASICs. Case and point, compare the computing power those GPUs will have against the (price)/(hashing power)/(power consumption) points of the 100 GH/s product.
With the current compounding rate of the difficulty factor, and the advertised end of February delivery date of the product, and current Bitcoin exchange rates, I'm not sure that I want to spend the money to buy this ASIC product because mining is likely to not be profitable. If current difficulty factors continue to increase at current rates, they will be at least three times the current 1,180,923,195 difficulty by early March. This compounding effect is not accounted for by calculators such as http://www.alloscomp.com/bitcoin/calculator.
Conclusion, Bitcoin mining is in a bubble unless the value of of the Bitcoin appreciates substantially. Stating that mining Bitcoins is in a bubble, does not imply the exchange rates for Bitcoins is in a bubble.
