If my Mini is running 64 bit can I run more than 4Gb RAM?
Is this a hardware limitaion of the motherboard/Processor?
Is this due to the fact that mini's run at 32 bit?
You can now easily run modern mini's at 64 bit:
So who knows the REAL answer?
Most of the Minis are limited to less than 4GB and that is a hardware limitation. The newest Mini with the nVidia video can access more. Apple only supports 4GB because they use matched DIMMs, which on paper have a slight performance improvement over unmatched DIMMs. For whatever reason, Apple doesn't yet support 4GB DIMMs in the Mini. (I suspect that the power requirements or prices for the larger DIMMS may have affected Apple's decision.)
If you do have an appropriate Mini, then you can get compatible 4GB DIMMs from [Macsales|http://eshop.macsales.com/shop/mac-mini> or others. Regardless, although I normally trust Macsales, more than 4GB is not supported by Apple, so you have to take that in to consideration. (i.e.: warranty from Apple or Applecare.)