As I understand it, it may depend upon your rack configuration as clbirk says 
and a business partner should be able to answer your question if you provide 
that to them.

But it depends on the model 520 installed - in earlier 520s memory was 
installed in quads and in the 520 Plus systems the memory was installed in 
pairs.   What is the model and processor feature of your box?

On older as/400's you could mix and match memory, yes you had to do it in 
pairs, but I had 32mb and 128mb on same mb.

IBM has a physical planning book like for V5R4 and it will talk about memory, 
etc. and I find it helpful. If you are buying new memory, the var or bp or 
whatever they are called these days has a a configuator that should tell you 
that answer also.

"Most systems today"...  It usually depends upon the architecture of the 
processor/motherboard and the memory cards.  If the processor is 64b and the 
memory cards are 32b, then you need to install in pairs.

The first PC I owned had a bank of 4 slots, and 2 banks of singles (longer).  
In the 1st bank I was required to run 4 simms of the same type, but the other 
2 banks could run anything I wanted (to the limits of the MB).

Right, don't empty anything you won't fill with new SIMMS.
Most systems today (don't know if POWER is the same way) require pairs of memory
SIMMS--although some newer systems do not require them to match any longer (only
that there are pairs, for example, in my MacBook Pro I can have a 3GB SIMM in
one slot and a 2GB SIMM in the other). But I don't know if POWER allows that,
however, having said that, it is the best system on Earth, so it probably does.

Pretty sure you can mix them. Cannot hurt to try. 
-Paul

The budget for additional memory has finally been approved.
We have 8 slots (model 520 on v5r4) – the boss decided to buy 2 '4 gig' chips.  
Doubling our memory.
But I am curious, can I keep 6 of the 512 meg chips in the machine, giving us 
11 gig.
Or is there no mixing memory sizes?