XP and 4GB RAM
The problem lies with how Windows XP allocates physical address space for devices. As you all know, XP is a 32-bit Operating System and has a total of 4gb memory space. Hardware devices will take I/O address resources from this 4GB. For Example, the 3.75gb~4.00gb (256mb) memory space is reserved for I/O APIC and BIOS EPROM; 3.50gb~3.75gb is reserved for configuration-space memory-mapping access for PCI Express, 3.25gb~-3.50gb is reserved for PCI Bridge devices such as the IDE Controller, USB Devices, and Onboard audio. When you have a PCI Express graphics card with 256mb of ram, 3gb~3.25gb will be allocated to the first PCI Express graphics card. A second PCI-E card would take another 256mb in the 2.75gb~out of 4gb of memory available for Windows applications. Memory remapping could be a solution, but theres a performance cost and some drivers might fail with the physical address extension enabled. The easy way around this is Windows XP 64-bit OS.