PC2100 instead of PC2700?

No it is not fast enough for your iMac.

Duane, not to question you, but I know that you can use different speed RAM than what your Mac calls for. Lots of people do it. Many people have used PC100 instead of PC133 as well as slower DDR. Is there something that I am missing?

As far as I know, yes, you can use PC2100 in a Mac that asks for PC2700, but it will not give you the (slight) speed advantage. However, it could cause issues related to the difference between the front side bus and the speed of the RAM as Macs are more sensitive to things like that.

The price difference between the 2100 and 2700 is not that great. Unless you are just using the slower RAM as a stopgap measure because you have it lying around, I would opt for the RAM that meets your Mac's specs.

I think you will find that people use PC133 instead of PC100 not the other way around. This is because the underlying bus speed is 100 MHz. Many PC133 devices can operate at the slower 100 MHz speed.

The iMac developer hardware note clearly states that PC2700 RAM is required for the 1.25 GHz iMac.

You can always try PC2100. It will either work, not work, or result in fairly unstable behavior. Trying it shouldn't do any physical damage but files may become corrupted if they are not read/written to RAM properly.

The price difference between the 2100 and 2700 is not that great.

In many instances it appears that PC2700 is less expensive.

That makes sense Duane. I think we ultimately said similar things - it is likely to work, but cause more errors and other potential problems and to buy the right spec RAM.

Thanks for clearing that up for me.

If by chance, you bought a high quality PC2100 DDR266 133MHz the best
you can hope for is it essentially may be similar to one of the PC2700/2100
series and was labelled PC2100 for marketing purposes.

Since PC2700 and PC2100 RAM are nearly the same cost per MB, there is
a small difference in the chip itself; designed for different bus speeds and
data transfer rates, the 2100 chip (as stated) may cause some data corr-
pution or other issue at some time. The better bet is to get proper RAM and
know that part of an upgrade hasn't the potential to be the cause of an
issue which could mess up something important you may be working on.

Manufacturer websites have some general information comparing RAM
types; the configuration pages are helpful to a point in getting the parts
which are recommended. Here's some basics on the topic:

See Mac Buyer Guide:
http://www.crucial.com/library/guides/mac/page03.asp

DDR RAM types, speeds, compatibility compared:
http://www.crucial.com/library/guides/mac/page03.asp#4

If the package your PC2100 RAM came in is new enough, it may say
somewhere if it was PC2700/2100 compatible. I've learned the hard way,
after a RAM upgrade, that even 2700/2100 RAM can crater in a 2700
machine; multi-tasking can push it too far and odd things start to happen.
The only time I ever paid someone professional to do a factory-RAM
slot upgrade (iMacG4 under warranty) and it failed big time; regretfully!

Best of luck in your experiment; it may do OK until you run three or
four applications at once and maybe run short on virtual memory, too.

 iBookG4 1.33 12" 1.5GB RAM /  iMacG4 1.25 17" 1GB RAM Mac OS X (10.4.6) & 10.3.9; w/ 2-  AirPort Extreme w/56k, on DSL + ext antenna

That may be a hard generic tool to find; unless a tech specialist happens to know and use something... which may be possible, but outside of most user's experience or availability.

I'm not sure to what extent and accuracy the Apple Hardware Test, which accompanies most modern Macs, would be able to say about your RAM type, speed, or other specifications. It may be able to give you a little more insight than the information brought forward in the less technical System Profiler utility. You should be able to boot the A.H.T. from either the OS X system DVD or separate CD, that came with your computer in the software packet of discs.

Some professional shops in the past had electronic equipment that could test individual RAM chips to see if they were up to specification and to see what they really were; given the fact some of these have little outwardly identifying information to the layman or non-professional; and they are hard to diagnose without electronics. That's probably why it is easier to just get a chip replaced (eventually) under warranty than try to get an actual RAM chip tested at any retail shop. The low-end test has been to swap out the suspect chip with a known-good one...!

Perhaps there is an OSX utility (other than ones like MemTest, etc) that can actually tell the user what speed the chip runs at, and be able to identify the specifications; as per documented specs vs. actual ones.

From one unhappy experience with lower cost RAM, I found it is better and in the long run cheaper, to be sure and get RAM from a reputable dealer who knows that Macs will work better, longer, with quality parts. The prime RAM slot would more likely discover lower-spec RAM, as the machine sees and runs that RAM primarily; and the second RAM slot may be able to get by under lighter use with poorer or marginal spec RAM; unless a newer G5 iMac or another one requiring matched RAM is brought into the question, then both should be equally balanced in use and in load specification, among other minimun specifications per OEM.

PS: The bus speed and speed of the RAM should be similar; but as was stated in more than one way earlier, you can sometimes install a faster speed RAM on a slower speed computer system bus, and it will work; this is, if the rest of the equation is equal.

In your case, as I suggested, it just may be the specification of your PC2100 chip may be really a backward compatible PC2700/2100. However, there are professional RAM testing methods (cost money) out there. You may see if you can get a rise out of the manufacturer as to how that specific part number (quote package) could show as a faster RAM in this situation; if you can get a person at their CR department!

For more general RAM FAQs, see this article. Right now, it has a new iPod+iTunes commercial, too. 🙂
http://www.macworld.com/2006/02/features/speedram/index.php

I'd tried to say you are welcome, in edit to my earlier post, but it seemed to have timed out. So here I am again. Best of luck & you are welcome!