In a 32-bit OS the system can only Address 4GB of memory. Your RAM, video card, and any pci card takes up address space. The 4870x2 has 2GB of cloned memory. Each core gets 1GB of memory that is an exact copy of the other 1GB of memory. The gpus can't access the same memory at the same time, so they came up with the 2GB cloned idea. Your video card has 256x2 bit interface(256 per gpu). If you didn't have a video card you would have 3.5GB of ram. If you added another 4870x2 you would have 2.5GB of ram. Hope this answered your question.