Hi,
First I would reseat the RAM. Pull both sticks out and put them back in, and try and boot. Then, put one stick in and try and boot, and do the same with the other. Also, do you know what the previous owner did? If (s)he removed the heatsink and didn't put on any Arctic Sliver 5, then you will have issues. If you know the owner did remove the heatsink, then I would remove it, remove the thermal pads, and apply arctic silver 5.
http://www.arcticsilver.com/ins_route_step2amdas5.html