I have a AMD Athlon 64 3800 X2, with a Asrock Dual SATA 2 motherboard, and 2x512 Kingston Hyper X running at 400 Mhz, CL2 in Dual channel mode. My computer ran fine until i decided to add another 2x512 Hyper X. When i run at this speed the computer will boot up, but when in Windows it starts becoming unstable, reseting etc.. So i set the bios to factory defaults, which the memory runs at 333 with no problems. If it runs fine with 1 gig of memory at 400, why can't i run 2 gig at the same speed. Please help!

???

Ok.. First you need to find out if the new ram you have added is faulty.

Put the 2 new ram sticks exactly how the old ones were (same slots) and leave the old ones out... So you only have 1GB in. If it is still usntable then one / both of your new ram chips are faulty. If not post back.

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It's not unusual for larger quantities of memory to have difficulty achieving the same latencies as smaller quantities.

If you have manually set the latencies (CAS, RAS tRCD etc), try setting the BIOS to auto or "Configure by SPD". You could also try changing the command latency to 2T from 1T.

If you want to test properly for errors (which I recommend under any circumstances when adding memory to a system), download and run Memtest86+.

;D Bingo!

I set the CAS to auto, it's now running fine, thanks

My pleasure