Firstly can your mobo take the 3GB of ram ?
Could you post the mobo details please,you can find these by running either Belarc or SIW
Belarc click here
SIW click here
OR PC Wizard click here
Alternatively if you are sure the mobo can run 3GB of ram,maybe one of the sticks of ram is faulty.
Try running Memtest to diagnose the ram for errors.
Direct download for bootable Memtest click here
Download and burn to disc,insert the disc and reboot the computer,leave Memtest to run for at least a couple of hours.