PC RAM upgrade - pulling RAM from different machines

I have an older IBM PC that has 512 and 256 Ram cards and a HP Pavillion PC that has 512MB.   Below are the details:

PC 1 - IBM
RAM 1 - 256MB DDR PC2100-25331-Z CL2.5
RAM 2 - 512MB DDR PC2700U-25330 CL2.5

PC 2 - HP Pavilion
RAM 1 - 512MB 1Rx8 PC2-4200U-444-11-A1

I wanted to combine the two 512 RAM cards into the HP Pavilion machine.  However, it will not boot when I put these cards in the machine.  I then tried putting the two RAM cards from my IBM machine into the HP Pavilion to realize 768 RAM and that didn't work either.  

Does anyone have thoughts or suggestions?  I also might go out and buy a 1GB module as well, so does anyone have suggestions about the best type of RAM to get?  Also, what does all these RAM numbers mean?  

Thanks!
rmnp0829Asked:
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johnb6767Connect With a Mentor Commented:
Check the mfgr. site for the specs on the MAX RAM for the board, and the max supported speeds. And if the HP is Dual channel, buy in pairs, dont mix and match brands....

Since it is one of the single most crucial parts of the system, dont buy the cheap stuff...

OCZ, Kingston and crucial are some of the top ones.
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johnb6767Commented:
Different speeds......Not good to mix them....

I have seen instances where sometimes they would boot mixed up, and would drop to the lowest speeds, but there were problems in windows at times.

512MB 1Rx8 PC2-4200U-444-11-A1
Isnt this dual channel?
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Lee W, MVPConnect With a Mentor Technology and Business Process AdvisorCommented:
Mixing different speeds provided they are the same type of RAM (DDR and DDR, DDR2 and DDR2) is generally not problem, but they do have to be the same general kind - both non-ECC or ECC, depending on what the board supports for example.

Technically, it shouldn't be possible on most boards to mix DDR and DDR2 as the pin configuration is different.

Would help if you posted a manufacturer for the RAM  The information you did post tells us little more than the speed (4200 vs 2700 vs 2100) and for the IBM, the CAS latency.

RAM can be tricky... if you want to get it right, I suggest you go to an online vendor such as Kingston (www.kingston.com) or Crucial (www.crucial.com) and use their memory selectors to buy appropriate RAM for the machine... even if you don't want to buy it online, you can see what kind it takes, print the sheet and bring it to a store to buy more there (and see how much more you're overpaying at the store).
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zephyr_hex (Megan)Connect With a Mentor DeveloperCommented:
it looks like your HP machine takes PC2 (DDR2), which is different than PC (DDR).

if you look at the DDR2 memory against the DDR memory, you will see a difference in the number of pins.

you will need to get PC2 (DDR2) memory to upgrade your HP machine.  it looks like your HP machine is relatively new (since it takes DDR2... i would guess that it's less than 2 years old)... and newer computers usually don't complain when you mix speeds of RAM.  it's the older machines that tend to have problems (i.e. computers that take PC2100 or lower...and are 4+ years old).  with those older types of memory, you also have to be aware of the ECC/non-ECC... but like i said... it shouldn't be a problem on your HP computer.  your problem is that DDR won't work on a DDR2 motherboard.
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KenneniahCommented:
PC 1 - IBM
RAM 1 - 256MB DDR PC2100-25331-Z CL2.5
RAM 2 - 512MB DDR PC2700U-25330 CL2.5

PC 2 - HP Pavilion
RAM 1 - 512MB 1Rx8 PC2-4200U-444-11-A1


The PC2100 and PC2700 are both DDR
PC2-4200 is DDR2 (there is a PC4200 that's DDR, but the PC2-4200 spec is DDR2), so no, you cannot mix them.
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KenneniahCommented:
Heh, which is basically what zephyr said....so ignore me :P
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