If bootup without the logo doesn't tell you anything, then I'd try taking out the extra RAM and seeing if it starts up with the original configuration. If it does, swap out the old RAM for the new RAM and see if it still starts up. If it does, try all the sticks together again.

I've had similar problems, and it turned out to be a dead stick. I'm not saying that that's your problem, though. From experience, RAM is the first thing I'd look at, particularly seeing as you upgraded it (yes, I know it was months ago, but that's what happened with mine).