PL1600 Parity error

[Gabrielle Nohra]

Hello,
I have a proliant PL1600R. 2 weeks ago I received a blue screen with following error:
Hardware Mulfunction. NMI: Parity Check/Memory Parity Error. The same error occured 3 times the same day.
Thinking that memory chips might have gone wrong, I went and bought 2 chips of part # CPQ-317749-001 (2 x 256 MB RAM).
After installation, all went fine for one week. Today morning, I received the same blue screen with the same error.
The server OS is windows 2000 advanced server.
It has been running smoothly for the last couple years.

What could be wrong?

Please help
Gabrielle