Power Supply with RAMPS 1.4 and heatbed question.

I am building a Prusa Mendel V2 metric in the USA.

I have an ATX12V style power supply, with 6 Amp and 10 Amp rails at 12 volt rating.
I have a MK2 PCB hotbed that I plan to used.

Is the 10 Amp leg enought for the hotbed? (The RAMPS 1.4 page says I should have 11 Amps)
Am I short one (1) Amp... I mean, what's an Amp between friends?

If I need a different supply, how accurate must the 12 volt be?

Is an unregulated DC supply ok, or is that asking for trouble?

Thanks in advance,

Richard