If the other suggestions don't help, in general, in cases like this, it's useful, but tedious, to go back to the original configuration to eliminate variables that could be causing the problem.
So if all else fails, pull the new RAM, make sure none of the heat sink grease you applied has run off onto electrical connections (it's usually electrically conductive as well as thermally conductive, I killed a PCC Power 120 logic board by overapplying heat sink grease, once), remove the hard drive (no need to install the old one) and try again. If you a good start up bong and the screen displays, then do it all again, and reinstall the RAM. If it's still good, reinstall the hard drive.
Like I said, tedious, but sometimes the only way to narrow a problem down to one culprit.
If the heat sink grease has gotten into the connectors, pull the whole circuit board, if you can, and thoroughly wash it to remove all the excess grease. A can of spray circuit board cleaner is handy in those types of situations, such as Flux-Off.
So if all else fails, pull the new RAM, make sure none of the heat sink grease you applied has run off onto electrical connections (it's usually electrically conductive as well as thermally conductive, I killed a PCC Power 120 logic board by overapplying heat sink grease, once), remove the hard drive (no need to install the old one) and try again. If you a good start up bong and the screen displays, then do it all again, and reinstall the RAM. If it's still good, reinstall the hard drive.
Like I said, tedious, but sometimes the only way to narrow a problem down to one culprit.
If the heat sink grease has gotten into the connectors, pull the whole circuit board, if you can, and thoroughly wash it to remove all the excess grease. A can of spray circuit board cleaner is handy in those types of situations, such as Flux-Off.