Maximum RAM? Different RAM?

I have bought and installed a stick of 512 MBs of SDRAM, placed it next to the 128 I already had, and booted up.

And the compy decided to just detect 256 MBs instead.

Before I turn everything off and unplug the box and take it off its (snug) emplacement all over again, here are my two questions while I try to search on Google:

A) How much RAM can a motherboard actually take?  More or less than Windows 2000 Pro would?  I'm using a Dell Dimension L933r, x86 Family 6 Model 8 Stepping 6.  It has two RAM slots.
B) Can I shove two sticks of SDRAM of different capacity next to each other?

I'd love to include info about banks and such but I don't know if the slots are separate banks or are paired as a single bank.  It originally housed a stick of 128 MB, I added another stick of 128 MB later on and now got that stick of 512 to replace one of the two (so I assume each slot is its own bank...)

Well, enough rambling.  >_>  Back to Google and thanks in advance.

A) This is dependent on your specific motherboard.  I don't know the specifics for your Dell model so Google around.
B) The manuals for most of the motherboards I've seen recommend using same capicity sticks for the banks but I've never had a problem using different capacity sticks.  You can always bite the bullet and just get another 512 stick... 1G is pretty nice.

Well, after some testing and Googling, here were my findings:

-My system won't go past 128 MBs in either slot, which means 256 is the best I can and will ever get.  Surprizing for a system bought in 2002.
-Googling revealed my system can actually take 512 MBs total!  And it can be in any configuration I want.  Why the system in reality doesn't comply with the figures is beyond me...

Guess that calls for a tech support inquiry.  I dread that moment.  >_<