I have an ASUS PR-DLS533 MB, capable of up to 12 GB. I had 2 x 512KB sticks originally. RAM reads 1 GB. PC2100, ECC Registered DDR.
I bought 3 more 1 GB sticks (Brand A) and a single 1 GB stick (Brand B)
I installed, in pairs, two of the A sticks. Ram now reads 3 GB.
I installed, in pairs but not matched, stick A and Stick B.
RAM now reads 2.5 GB!
I have swapped the 1's all around, and it is definitely stick B that is the problem. My question is why?
Any thoughts?

Just check the following... you're trying to get 5 GB right, maybe the proble is that you have 4 1GB and 2 512, try the 4 GB alone, and see if that gives you 4 GB. It could be that you are interchanging with 1GB and 512.
Try that, I'm curious if that's it.
If not, I have a problem once where a slot was not working, I have 2 slots and I had 512 in each to make 1GB, but it only read the 512, it turned out it was a slot, but the machine still worked, kinda strange, but it happens.

server boards are really picky about ram. it is best to keep all the same brand and size chips with in the board. i would never mix and match brands in there either.

I have an old proliant server . It can get a max ram of 1 GB BUT WITH NO MORE THAN 512MB IN EACH SLOT- Check if yours is like this