pc133 ram in pc100 mobo

My wife has an AI61 Shuttle recently upgraded to Windows XP. Added more RAM up to 3 sticks of 128MB for total of 384MB RAM. Tried to get a little more so I popped in a 256MB stick of PC133 along with two others. Bottom line, it doesn't recognize any more than 256MB RAM. Has anyone else had this trouble. Is there a chance I would do better with 256MB PC100 sticks? The manual says it can take 3 sticks of 256MB RAM for a total of 768, but I can't get any higher than 384 with the 128 sticks, or 256MB with any combination of 256 sticks along with others. Some of the 128 sticks are PC133.

it's the density of the memory chips, not the speed rating. If you have a 256MB Dimm with only 8 chips, the board will only see 1/2 the amount. It would have to be a 16 chip 256MB dimm.

Thanks, Midknyte,

I got two low density double-sided 256MB RAM sticks. The Shuttle AI61 recognizes them both. The memory has been doubled from 320MB to 640MB. Her Windows XP should function a little better now. At least it should be worth the $10 spent.