Use 3GB or 4GB RAM in 32 bit OS?

Hi all,
I own a Dell dimension 8400 with 1 GB of RAM running windows XP SP 2
and also just bought a Gateway ML6720 laptop with 1 GB of RAM running
Windows Vista home premium.  I believe the Dell machine as for memory
slots in the Gateway 82 memory slots.  I'd like to bring both these
machines up to 4 GB of RAM but read this isn't possible in a machine
running a 32-bit OS, I read this at
http://www.crucial.com/systemscanner/index.aspx.
It said, "How much
memory your Windows OS will recognize depends on which version of
Windows you are running. 32-bit versions of Windows will see (and
utilize) only 3GB or 3.5GB. To utilize more memory, install a 64-bit
version of your OS." My question is, can I install 2 GB in one slot
and 1 GB in the other available **** to make up the 3 GB or would I
have to install 2 GB of RAM in each available slot even though the OS
would only recognize 3 GB?  I ask because a friend of mine said that
mixing RAM chip sizes really impedes the performance of a PC. Also,
some chips are labeled PC2-3200 and PC2-6400 etc. I understand these
denote RAM speeds? I'll be using Dragon Dictate heavily on both
machines and am wondering how big a difference it would make in
performance to get chips with a very high speed rating such as
PC2-6400?  Thanks in advance for any info you can provide!