System: Motherboard = Gigabyte GA-EP45C-UD3R OS = Win 7 - 64 bit Memory = CRUCIAL CT2KIT12864AA1067 (2-1GB DDR2 8500) I Purchased a new set of DDR2 DIMMs, CRUCIAL CT2KIT25664AA1067 (2-2GB DDR2 8500). Original memory was installed in slots DDR2_1 and DDR2_3 per manual for a total of 2GB. I installed the new memory in slots DDR2_2 and DDR2_4. This should create two matched sets for a total of 6GB. On boot the PC started rebooting at windows load. I removed the two 1GB modules and installed the new 2GB modules one at a time and then together in the DDR2_1 and DDR2_3 slots. Each time the PC booted fine. I then installed the original 1GB DIMMs in slots DDR2_2 and DDR2_4, (the reverse of my first attempt) This time the PC would reboot without completing POST. The BIOS had a continuous series of beeps between boots. Both sets of DIMMs are the same except for memory size, ( DDR2 PC2-8500 • CL=7 • Unbuffered • NON-ECC • DDR2-1066 • 1.8V). What have I missed?