Hi,

You might try to repair the file permissions & to rebuild the Extensions Cache.

If that doesn't work, maybe your old DSDT.aml file does contain any fixes that had been necessary for your Snow Leopard Installation to boot up properly.
So we might try to fix your working DSDT to support your new set of RAM.

As your 10.5.8 disk boots properly with your new set of RAM, use it to extract a DSDT when using your new RAM - dissamble both, the old one and the one created when using your new RAM and have a look at them.

There should be at least one line (maybe more) that look like this (it's taken from my systems DSDT table) :

OperationRegion (BIOS, SystemMemory, 0x3FFC0064, 0xFF)

See if you can adjust those values in your old DSDT to match those of your new RAM, then reassemble.

Good Luck.

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