kengdahl:
I've used the circuit quite a few times. If it is a quality built Ramps board; which I believe yours is, there should be no problems hooking up a heated bed to it. Be aware that the mosfet dedicated to the heatbed does get hot and the board is not overly cooled in the materia101 design.
I have the standard board supplied by Arduino, the 101 arrived last week and has produced its first object earlier this week.
The heater I intend to use will take about 3Amps at 12 Volt and is capable of heating up to around 60C.
I will be using a separate 12Volt supply for the heater fitting an extra socket to ensure the existing socket is not overloaded and a relay switched from the existing 12V supply to isolate the new 12 volt supply from the main on/off switch, I will also fit heat sinks to the MosFets.
I also intend to replace the glass plate with a similar sized piece of copper clad board using the copper surface as the bed with the usual blue tape. The copper is a good heat conductor so should even out any hot spots and also has the advantage that is can be sensed by an inductive sensor so I can use it with auto bed leveling, using an M5 sensor fitted into the extruder head.
I think a bigger fan is a must as the existing fan does not appear to draw much air into the base of the 101, I am thinking of using a 12V larger diameter unit.