Ok I posted on this particular matter once before, but I can't find the thread.
Basically it works like this.
Memory being sold today in the PC133 variety has different desities. This means they are physically different in size/#chips on the memory module.
Now, take for instance a 256 pc133 module. if it has only 8 chips on one side then it is the new high density chips. If it has 8 chips on both sides, then it is the lower density chips.
These different density chips may not function together in the same system. Even if you buy the SAME density chiped memory module for an upgrade to an existing module you still take a chance of the two sticks not being compatible. Also some of the older MTB's may not work with the higher density chips.
Here's a quick chart for ya:
256 PC133 w-8chips/side = low density
256 PC133 w-8 chips-1 side = high density
128 PC133 w-8chips- 1 side = low density
128 PC133 w-4 chips- 1 side = high density
I've also seen (very rare) 256 PC133 w 16chips-1 side = low density.
Hope this helps...