make sure all your ram you're trying to use matches the requirements....
Dual channel memory architecture
Four 240-pin DDR2 DIMM sockets
Supports PC-4200 (533 MHz), PC2-5300 (667 MHz), and PC-6400 (800 MHz) DDR2 DIMMs
Non-ECC memory only, unbuffered
supports 2GB DDR2 DIMMs
Supports up to 8 GB on 64 bit PCs
Supports up to 4 GB* on 32 bit PCs