Help Please dell 8400 memory problems

I started with a 8200 and bought a 8400 mb, 1g PNY ddr 2 5200, gforce 6600 256, and when it all came together i get gygy on the LEDs and a beep code that registers as memory error. i have tried the memory in another pc (core duo E, 4g PNY 5200, gforce 7200 512) and it worked fine. i have reset the bios and pulled the battery and still get the same results. really want to keep the 8400 because i have really moded the case with water cooling, window and lights.