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I've seen many a Mac startup and fail to boot / Spinny wheel and its been the RAM at fault, same as with a windows PC if the rams at fault it can successfully boot windows and will randomly BSOD, depends if its the entire RAM module or just one of the chips!
I have a fair bit of experience messing around with Bootloaders etc, and the constant spinning wheel in my experience is often down to missing strings in the boot.plist, if you could get past the spinning wheel you may be able to mess around with/restore your old .plist?
I would download iBoot and use that to try and get past the spinning wheel.
EDIT: Failing all that, it's probably a hardware compatibility issue, but again you won't really be able to resolve it without getting past the spinning circle.
Have a Google, I'm sure there's a memtest for Mac if not a built in way of testing the RAM, sorry I don't work on macs as much as I do windows machines so I'm a bit vague!