As Jeremy Clarkson (presenter of BBC car show "Top Gear") would say: "There's no such thing as too much power!" As long as you only hook up a maximum of 12 V (i.e. don't short the mains voltage to the 12 V side, and don't hook the -12 V to Ground, coz you might get 24 V total!), and connect to your RAMPS with correct polarity, you can't really "overpower" the RAMPS, and your unit has enough power to be able to power the Heat Bed as well. Your power supply is rated to 650 W, but it will only actually deliver as much power as the load demands.
With respect to spare wires - bundle together enough wires of the same colour to provide the power / current supply capacity you need, and use these, and just cut off any left-overs and seal the ends with insulation.
If you follow the instructions on the Wiki [
www.wikihow.com], you end up with binding posts to attach your leads to as a desktop laboratory power supply unit. In my case [
julianh72.blogspot.com] , I left the spare wires attached projecting through the hole at the back of the unit, insulated them and wound them into a neat bunch at the back of the unit, in case I ever want to access them for other purposes, but I think my unit will mostly end up being a dedicated power supply for my RepRap, so I will probably never need to access them. The "official" Wiki version of the ATX Power Supply installs binding posts for +/- 12 V and +/- 5 V - but you actually only need Ground and +12 V for RAMPS; the others may be useful for a general purpose electronics power supply. I brought out the +3.3 V line as well - just because I could, and it might come in handy some day.
Most power supplies come in a quite presentable box anyway, and it "looks the part" for a home-built 3D printer. (I don't
WANT my RepRap to look like I bought it from a shop!) It seems a bit odd to put a box in another box (unless you are a fan of Russian Dolls). If you do package it into another container, make sure to leave the ventilation free, or it could overheat very quickly.
The connectors go by a variety of names - headers, connectors, etc - and there are dozens (hundreds? thousands?) of different varieties. Most come in male and female halves; some are soldered, some are crimped, etc. Go to your local electronics / hobby shop to see what they carry. my local (in Australia) is JayCar [
www.jaycar.com.au]
Follow my Mendel Prusa build here: [
julianh72.blogspot.com]