since these guys are giving you hard time on accuracy, I'll help you out. the answer is 42.
Steve and Gerry are right; asking about accuracy is like asking how long is a piece of string...and there advice is good. build away and by end of the project you will understand why there is not a simple answer and you'll start thinking of accuracy in terms of repeatability, resolution, backlash compensation, feed screw quality, bearing quality, deflection under load, etc etc.
As someone else who is also on the cnc learning curve, don't get stressed figuring it all out in one night. to be really really good at this from diy perspective requires fairly advance electronics and machining/mechanical knowledge and abilities, these take time to develop as its a lucky few that already have both nailed down. a project's the best way accelerate up the curve imo.
having said that, if there is some parameter that you just can't live without, try to better define it, but remember that from cutting foam to metal you've just about covered the spectrum so be reasonable with your first machine expectations