I've seen this one a dozen times...
AGP connectors are notorious for not holding the board firmly enough and when you worked on the motherboard, flexing it to get the memory in, you probably pulled the video card half way out of the connector.
Turn off your computer, take out the video card... clean the AGP socket with some alcohol and a soft toothbrush, clean the edge of the board with a soft pencil eraser (the pink kind) and put it all back together.
50/50 that's gonna fix your problem.
--->It ain't better if it don't work<---