I'd take out the battery while testing, and then reseat the RAM. What color is the light on the AC adapter plug? With the battery out, it should be solid green. If there is no light, if it's dim green, or it flickers, that's a problem and could point to a problem with the DC-in board, or other related connections. Or, you AC adapter might have chosen to coincidentally flake out simultaneously with putting in the new drive, so testing it with another AC couldn't hurt.
I would keep the machine opened up, and try powering on while applying pressure to the topcase's connection to the logic board. There is enough slack in the cable to do this, and it makes life easier to keep the topcase disconnected until you know it powers on.
If the AC adapter light is green, and none of this works, I'd look into jumping the logic board. I believe this model has two connectors on the board you can use jump the power (ruling out the topcase as the problem), although I'm traveling an I don't have access to the service manuals in order to look up the location on the board.
As Charles points out, hard drive replacement should not cause a power issue. You might want to remove the hard drive and drive cable entirely while testing...any MacBook should power on just fine to the blinking question mark while its hard drive is out.