i had a weird issue like this, i went out and bought 4GB of the exact same RAM i had, and my computer would lock up at the POST screen.
my board supported 4GB, and the exact model RAM worked in there before.
what i did was put in 2GB, reformatted and reinstalled OS, shut down, put 4GB in and then rebooted.... prob solved.
every once in a while, it'll reboot and not like the RAM, and i had to reboot with 1 stick @ 2GB, then shut down and add the other @ 2GB. don't ask me why, the DIMMs run individually just fine... it's only when they are both in.