AboveDaClouds,
It is my understanding that the 4Gb limit on the Dimension 8400 is a limitation on the chipset / motherboard and also that this 4Gb limit is a limit for all the memory addressed (including video card and other stuff), not just the ram. So even using a 64-bit OS will not enable you to use all 4Gb ram. (I am 99% certain I even read a thread on here a while ago where someone asked why when they upgraded to a 64-bit OS on the 8400 that the full 4Gb ram wasn't visible and this was the verified answer given).
