Can't get all memory modules (RAM) seen.

Hi all, I have an early 2008 Mac Pro - 2 x quad core 2.8Ghz processors, its the model (pic below).

I have the following memory modules - all Kingston:

2 x 512Mb PC2-5300F (1RX8)

2 x 1Gb PC2-5300F (1RX8)

2 x 2Gb PC2-5300F (2RX8)

2 x 1Gb PC2-6400F (1RX8)

I can only get a maximum of 4Gb picked up using any combination of the modules above. I put them in pairs, so, on each memory board, I will use identical module placements on each board. The 2Gb modules only seem to work by themselves, when I put the 1Gb modules in with them, the system says I have 4Gb RAM - made up of 4 x 1Gb modules !! When I use all the 1Gb and 512Mb modules I still only get 4Gb show up - even though, when each module is tested as single pairs, ALL the modules work and no red lights appear on the memory board.

Can someone shed some light on this please. My ultimate goal is to get as much RAM as I can, ideally 8Gb at least, I will happily buy a 4Gb upgrade if need be, but I don't know if they would work with the 2 x 2Gb modules I currently have and also the smaller modules.

Thanks