If I have two or more memory modules, can I get a system to use them in RAID-1 ("RAIM"?) as a poor-man's ECC?
It is possible to use RAM in something like RAID1 mode - with many server mainboards (at least IBM and HP), that provide this feature under different names, "Memory Mirroring" being the one I most often read.
I do not think, that any of these will work with non-ECC RAM: If the two memory modules disagree about the value stored at an address, and no ECC exists, how should the system know, which of the values to use as correct, and which to discard as wrong?
This is a classic "split-brain" problem, that is hard to solve whenever a cluster of exactly 2 members work together. Things are easier in storage: Disks have their ECC built-in, so silent corruption without a read failure or other detection is less likely by orders of magnitude.