PC Turning On Issue

I have the exact problem as of tonight with my son's hp pavilion 7920. He bought a 256mb dimm of sdram, which appears to be the correct type; he first installed it next to the existing memory, powered up to nothing. I have been working on it all evening; took the battery out, let it sit 30 minutes or so, replaced the original memory and removed the new one, same results. I am stuck, don't want to take it into some rip-off shop like others have. Has anyone found a solution yet? It must be the RAM, since it was working fine right up until the point where he installed it.

i'm pretty sure i re-seated everything possible. It does seem like power because the mouse, keyboard and monitor all stay off, while just the fan runs. I took out the video card because he's had trouble with it in the past; turned the power on, then the light came on in the front as though the hard drive was at least trying; put it back and no light. So maybe the video card got destroyed? but why wouldn't the mouse and keyboard at least come on? This is most perplexing. thanks for the advice, i'll check for fused components later.

I answered my own question; if the video card is fried, the boot sequence will hang while looking for video, right? so it won't proceed to discovering the mouse, keyboard, etc, thus they aren't coming on.

don't have another one that will fit into the socket. that's the next step, scavenging.

i remembered it has on board video, so took out the bad video card, hooked the monitor to the on board video and voila, it's booting and looks ok.
thanks for everyone's responses and help.