Computer RAM: PC100 and PC133 together?

My Intel based Gateway PC is a couple years old, with a PC100 DIMM, 128 meg.

I want to add another DIMM, 128 or 256.

Can I purchase a PC133, or do I have to stick with PC100 DIMMs?

(I’m asking the Teeming Millions because I’ve gotten conflicting answers at the various local computer stores.)

Well… even if it did work, you’d be spending extra money on a pc133 without receiving the benefits of the extra speed. So your best bet is to spend less money on the pc100.

Yes you can.

It would almost be a sin to add 128… ram is so dirt cheap you can add 512 MB for under $50…

and It should work but, I have also heard that as far as stability goes you should have the same type of RAM.

I would take out the old RAM and add 2 new 512 MB sticks.
It is only lie $100 and it would really speed up the computer

All the ads I have seen recently list PC100 as more expensive than PC133. It isn’t made much and production quantity directly relates to cost savings. has a list of what you can put in your computer. You input your computer & model & it gives a list of what you can use.

The RAM will run at the speed dictated by the front side bus of your motherboard, which sounds to me as if it’s 100 MHz, though if it is a Celeron-based system it may run at 66 MHz. At any rate, there should be no problem whatsoever running the RAM below its rating.

There are occasionally problems with two different sticks of RAM not getting along with one another, but these problems seem to be fairly rare when you’re not overclocking the system or otherwise running it out of spec.

      • About three years ago I bought 256 for a PC with 64-megs, and the shop told me that you can mix 100 and 133 - they only sold 133 back then, because it was cheaper/because their suppliers didn’t even carry PC100 anymore. You can put faster ram into a slower machine, just not the other way around.
  • The OP didn’t state their OS, but the shop also noted that with Win98 (which I had) upgrading to more than 512 megs RAM often caused problems. I saw a big improvement going from 64 to 296 megs, but of course everything’s relative: the CPU is only a PII 350Mhz.

I wouldn’t buy the 512 MB sticks. Especially if it’s a Gateway w/a 100mHz front-side bus speed. What’s the CPU? If it’s a PII, then I can guarantee that it won’t take more than 128MB sticks per slot (for a total of 384MB), unless it’s a higher-end business workstation (E-series). If it’s a low-end PIII processor, I am pretty sure those didn’t take more than 384MB total either.


On my ASUS p5a motherboard, some sticks of cheap pc133 only register as half the size they are rated at., i.e. a 256MB only operates as a 128MB. I don’t know why this is. Best to use’s memory selector.

Wow. I go to a couple of meetings and there are tons of replies.

Thanks all.

It’s a PIII 700 mhz, and I’m running Win98.

Does that change anyone’s opinion?

I did the EXACT same thing with the EXACT same machine. The guys at the shop said I should tear out the old 64/100 and install the two matching 128/133`s just to keep it balanced. The thing works great, alot faster than before.

Hmmm. Cruising the site tells me that this is more complicated than I thought.

Is there any performance difference between a single 512 and two 256s?

Oops. I didn’t read your post close enough critter42. If what you say is true, then I absolutely should go with two 256s vs a 512.

      • The computer I have came with one 64-meg PC100 card, and I added another 256-meg PC133 card. The memory cards didn’t need to be balanced, as it had three ram slots and only came with one ram card OEM.

My thanks to handy and stevephillips for the suggestion to use for guidance.

I went home last night to check the specs on my PC. Needless to say, the receipts from Gateway were less than precise. The site has a free downloadable utility to examine your machine to determine everything that is on it, both hardware and software.

It determined what my motherboard was, and then the site could tell me what memory was compatible.

Turns out I cannot put anything larger than 128s in each slot, and the maximum is indeed 384 as critter42 suspected. It also said that it didn’t matter if I bought PC100 or PC133 DIMMS, because either would work on my motherboard.

As a consequence of how helpful their site was, I just went ahead and ordered the memory from Who says customer service doesn’t pay? I bought two 128 PC133 DIMMs for $20 each (with free 2 day FedEx shipping!).

Thanks to all the Dopers who replied to my question. It was very helpful.

I think the term “balancing” is more of a phycological thing than a computer issue. The faster (133) chips are backwards compatible so adding one of those with an existing (100) should be just fine, I`ve been told. If the computer recognizes the install and things are running better than all is OK.

I had pc100 and pc133 memory mixed on my old athelon 550mhz machine with a via 501 MB. It was unstable. Running Win98 it crashed a lot and did all kinds of goofy things. All this went away when I yanked the pc100 memory and threw in a new pc133 stick.
I won’t mix memory like that again.

We just added some RAM and were interested to find out that the first pick, 133, physically didn’t fit in the slot (E-machines computer from 2001). So we went back and got 100 and it fit. And worked.