Mac Forum / General / General / December 2006

The maximum RAM supported in a 400 MHz iMac G3 is 1 GB (two 512 MB
PC-100 SDRAM DIMMs).

I ran about Mac OS X 10.1 on my 400 MHz iMac in 384 MB, and it was
reasonably OK. My user group still has that model, and we were running
10.3 in 256 MB, which was OK was long as we didn't run anything memory
hungry or try to run multiple applications. That wouldn't be enough for
10.4. I think we've upgraded the RAM since then but I haven't had a
close look at the computer for a while.

You could start by replacing one of the 64MB DIMMs with a 512MB. That
should be enough for most tasks. If the machine seems unbearably slow in
normal use and you can hear a lot of disk activity when switching
between applications, then you probably need more RAM and should
consider replacing the other 64 MB DIMM with a larger one.

There isn't much point getting 256 MB, apart from saving a few dollars.
You would probably just have to upgrade it again.

I have 768 MB in my PowerBook G4 and it used to be enough but is getting
too small as the operating system and applications keep getting larger
and wasting memory for no useful reason (lazy programming, bugs, or just
for pretty graphics).

PC133 memory is usually compatible with PC100. The only problem is with
cheaper memory which doesn't fully comply with the standards.

DIMMs contain a "serial presence detect" EEPROM (a small amount of
non-volatile memory) which is supposed to contain information to allow
the computer to identify the speed and size of the DIMM. The Mac depends
on this information being programmed correctly, and cheaper memory
sometimes omits all or some of this data. For PC133 to work in a PC100
machine, the EEPROM on the DIMM must contain settings for both 133 and
100 MHz bus speeds.

There are also some special types of memory which might not work in a
Mac but which also use "PC100" or "PC133" as part of their description.
In particular, there are two form factors: you want a normal size DIMM,
not a SO-DIMM. Some Mac models also have physical size restrictions on
the DIMM, due to lack of space (this shouldn't apply to your iMac).

Your safest bet is to buy memory from a dealer who will guarantee that
it will work on your particular Mac model, or who will accept a return
with no questions asked (if the memory doesn't work).

One option is Other World Computing <http://www.macsales.com>, who are a
specialist Mac dealer. They are selling memory which is described as
supporting both PC133 and PC100.

That is the specifications for either your model or the one immediately
before it, but they are almost identical. (I'm not sure whether you have
a "Slot Loading" model, which is late 1999, or a "Summer 2000" model,
which is mid 2000.)

That "406" number is meaningless. That is the machine ID using the old
numbering system that was used on 68K and early PowerMac models, now
known as "old world". All models starting with the iMac (with built-in
USB) have the same ID number, revealing it to be a "new world"
architecture. There is a separate string which identifies the general
model, and for your iMac it would be "PowerMac2,1" or "PowerMac2,2"
depending on whether your iMac is the late 1999 or mid 2000 model. In
some cases this string isn't sufficient to identify the specific model,
and you need to look at other characteristics to narrow it down further
(e.g. processor speed or type of graphics card).

Both of those iMac models require PC100 memory. They will only work with
PC133 memory if that memory is also configured to operate at the slower
PC100 speed.