Gonzalez: it's not the values of the voltages that matter. 4.8, 5, 5.2, doesn't matter -it's all perfectly +5 to the motherboard. The motherboard will in general be able to regulate the voltage to the proper value in any case. The only way that a lower voltage would possibly cause any problem is because at a lower voltage, the board may draw more current to draw an equivalent amount of power - however, honestly, the difference is minimal: 4.8V would draw 4% more current than a supply at 5V (assuming linearity and all that - not a good assumption in this case, but it only gets better if you consider the fact that the power regulators aren't ohmic). You could easily get away with a 250W power supply, if the rails can supply the right configuration of current, however, most power supplies are marginal, and so it's best to go with a 300W one.
On topic, however, I just got a hideaway case from them for $44.99+$17 shipping. They're a good company, but don't expect a massive amount of customer support - they can't do it. They're a tiny company, basically succeeding on the fact that they have no support staff. In my case, I got a confirmation letter from the president of the company telling me when my order would arrive.