Remember, RAM must be installed in pairs. You can only have 2 chips, or 4 chips, or 6 chips, or 8 chips.
And make sure you're putting the DIMMs in to this order:
4
3
2
1
1
2
3
4
If you only have two chips, put them both in the first slot of their respective bank.
As I mentioned in another thread, get a Q-tip with rubbing alcohol, go over the chips' contacts (carefully, so no hairs get left behind), go over the DIMM slots on the logic board (carefully), and reinstall them according to the map above, making a very firm connection in the process.
Boot it up, position your hands in front of you (pointing at the machine), and declare 'Open sesame!' for good measure. 🙂