I have just upgraded my 2009 MacBook RAM from 2GB to 8GB.
Everything works just fine whilst I am using it, things seem much snappier in general most of the time. The main problem occurs when I restart after hibernating. It will consistently crash and reboot the Mac. Also some high intensity tasks seems to now take ridiculous amounts of CPU. Compiling in XCode at times is much slower..
I spend a couple of days running MemTest, and no problems were found with the memory, so as far as I can tell I haven't been sold broken goods. It's got me thinking that perhaps something needs to be configured somewhere.
It's got me thinking that perhaps the increased RAM now means it has to create a bigger swap file and isn't able to.. pure speculation..