|According to Crucial, the system has 2 memory slots & can handle up to 512MB per slot. I assume you removed the 256MB & installed 2 x 512MB? Did you check the specs of the RAM? Voltage is very important. DDR-SDRAM is supposed to run at |1.8v but some manufacturer's spec it out at 1.9v, 2.0v, 2.2v, etc.
CORRECTION: DDR-SDRAM is supposed to run at 2.5v. The voltages I quoted above are for DDR2-SDRAM.
If you bought high-voltage RAM, it may not work properly in your system. Also, RAM should always be tested to confirm it's error-free before it's put into service. Defective RAM can cause all sorts of problems. Download memtest86 & create either a bootable floppy or CD, then boot off it & let it run. If any errors are found, replace the RAM.