Q: upgrading RAM will not boot

I have the same problem here with a 2007 model. I have two identical 2gig samsung modules that I'm trying to use to upgrade my mac from 2 to 4gig. Either 2gig module work fine when paired with one of the existing 1gig module. Both 2gig modules together and the computer does not start. I've tried every possible combination 2gig + 1gig to verify that it's not a problem with my modules or with a particular slot.

I was hoping their was a magical setting somewhere to allow the combination;-)

I also tried to upgrade a 7,1 (mid-2007) iMac with DDR2 800MHz 2 GB modues removed from a 2008 iMac. As a few here have noted, and as reflected in other threads, the 7,1 iMac specified 667MHz memory.

When one 800 Mhz module and 667MHz module are installed the iMac 7,1 computer System Information reports 3GB of 667MHz memory. (If two 800MHz modules are installed the computer won't boot.)

My question is: What sort of problems might I see if I use this configuration?

i am having the same issue.  did you ever get your problem solved?

Likely intermittent crashes as well as loss of speed when read/writing to the RAM. It would be better to use a matched pair of slower RAM than an unmatched pair with one stick being faster since they will only run at the slowest speed anyway. Read more here.

I am having a bit different problem.  bought two 2 gb of ram running at same speed. 667 for an iMac 07 2 dual core 2.4 ghz.  when i put in both sets, the computer won't boot up.  when i use one and pair with the old 1 gb set it boots fine and shows 3 gb of memory on profile. 

Can you embellish, I would be happy to assist. You installed 4 but get no boot, installing 3 works? Which iMac? What brand RAM?

Which size iMac just in case it matters?

here is the profile info and the brand is Centon

  Model Name:    iMac

  Model Identifier:    iMac7,1

  Processor Name:    Intel Core 2 Duo

  Processor Speed:    2.4 GHz

  Number Of Processors:    1

  Total Number Of Cores:    2

  L2 Cache:    4 MB

  Memory:    3 GB

  Bus Speed:    800 MHz

  Boot ROM Version:    IM71.007A.B03

  SMC Version (system):    1.21f4

  Serial Number (system):    W87469Z6X89

  Hardware UUID:    00000000-0000-1000-8000-001B63B8913A

500 gb capacity

sorry 24 as well just realized what you were asking

Do both chips work in either slot alone? I'd make sure one isn't bad first. I don't see any reason it can't work, however things that have been said about Apple machines being picky about RAM are very true. So the brand could be the issue, I know it sounds dumb. This machine was one of the first that were starting to get really bad about it. In case you care or need to double-check the exact apple recommended specs for that model is PC2-5300 (667 MHz) DDR2 SO-DIMM, Crucial and other sites are wrong, and although it could work it isn't what Apple actually recommended for the model.

No problem, see my above reply

i have not tried to boot with each alone.  I'll check it out.