Memory issue when installing RAM

What Motherboard do you have?

Here's the info I got from CPU-Z:

Motherboard
Manufacturer: ASUSTek Computer Inc.
Model: NAOS 1.05
Chipset: NVIDIA GetForce 6150 Rev. A2
Southbridge: NVIDIA nForce 410/430 MCP
LPCIO: SMSC

BIOS
Brand: Phoenix Technologies, LTD
Version: 3.00

Since the chip speed is double the cpu-z reading 257.7 x 2 = 515.4 mhz
The ddr2 533 chips run at 667 mhz or in cpu-z they would say 333 mhz.
If your mixing the faster 667 chips with some slower ones you might need to manualy set bios for chip speed to the lower value.
Problem is can't guess what kind of chip you have that runs at 515.4
Also try altering the position of each set on the motherboard.
Did you install a pair or just one chip of each? if one chip of each might need to put one in each separte channel.