For the power and reset switch problem, google is your friend. I found this website quickly which explains it lower down on the page.http://www.redhotscott.co.uk/nespc.html
Basically the switches need to be momentary, but the NES power button is toggle so you need to modify it as they say so. On your motherboard there should be a series of jumpers for the front panel, and you wire the switches to them. You need to look at the motherboard manual for the actual pinout, but it shouldn't be too difficult. I have never done any PC modding so take this advice accordingly (with a pinch of salt
As for the stepping down of current, you don't step down current (unless you are dealing with AC stuff), you limit current. I assume that your circuit draws a maximum of 2a and you have a 9a supply, that means that the supply can deal with much more current than your item will ever draw, so all is fine. I would to be safe use a fuse (between power supply and circuit) that is just above the 2a maximum current just in case your circuit tries to draw a lot more current than it should do (short circuit or something similar). Hopefully the fuse will blow before too much damage is done to the rest of the circuit.