Major upgrades fail with 6GB in iMac 2007

[AndyCalifornia]

I'll specify at the outset that:

• Correlation is not causation
• 6GB works fine in an iMac 2007
• I've done this on a dozen other iMacs

But I'm upgrading an iMac 2007 to 6GB and seeing something very odd. The machine had Snow Leopard and after putting in 6 GB (4 and 2 GB DDR2 5300's) I began the upgrade to ML. Failed repeatedly with errors I've seen discussed elsewhere about being unable to download needed updates from the internet. Eventually the system became unbootable and I wiped, went back to clean Snow Leopard and this time created a bootable ML thumb drive that did not need a download. Eventually I got successfully to ML.

Then, when everything was nearly ready I attempted to download a very large update to iPhoto '11, about 800 MB. Sure enough, it crashed halfway through and the system was once again unbootable.

I had to go (faster this time) back to SL, but this time I did ALL my upgrades with 3GB ram installed (2 + 1) and only went to 6GB when everything was updated. I'm worried though that the next time there's a large update to download the same thing's going to happen.

I know it sounds crazy, but has anyone experienced anything like this? I've alternately read that the position of the 4GB and 2GB does or does not matter, but those who say it matters all say 2GB in slot 0 and 4GB in slot 1, so I've switched mine to be in that order. But I'd love to hear from someone who's experienced this. Thanks!