Issues installing 8Gb memory on Early 2009 Mac mini

Hey,

I was hoping someone might be able to help. I've got an Early 2009 Mac mini with 4Gb of RAM in two 2Gb SO-DIMMs. I thought I'd upgrade to 8Gb and bought 2 x 4Gb 1066MHz SO-DIMMs from Blazing Trade Inc (PC3-8500 DDR3): actually I bought two sets (4 SO-DIMMs) so I could upgrade my wife's laptop along with an SSD.

Unfortunately, this doesn't seem to work: one 4Gb in the top slot works, but not in the bottom slot. No combination of memory seems to work: two cards that work in the top slot do not work in combination. I've installed EFI-Firmware-Update 1.2 previously. Is this likely to be an issue with the memory or simply an issue with the age of the computer?

Any help gratefully received. Many thanks in advance,

Sam