You can't stop it completely, otherwise none of those 3 video cards would work at all.
Here is an excellent description of how memory mapping works:
Various devices in a typical computer require memory-mapped access. This is known as memory-mapped I/O (MMIO). For the MMIO space to be available to 32-bit operating systems, the MMIO space must reside within the first 4 GB of address space.
For example, if you have a video card that has 256 MB of onboard memory, that memory must be mapped within the first 4 GB of address space. If 4 GB of system memory is already installed, part of that address space must be reserved by the graphics memory mapping. Graphics memory mapping overwrites a part of the system memory. These conditions reduce the total amount of system memory that is available to the operating system.
While the same principle applies to computers with 4GB of total system ram, running 64 bit operating systems, the problem of "losing" ram due to how 32 bit operating systems must map it, is avoided by doing the following:
1. Installing and running a 64 bit operating system
2. Using a motherboard with a BIOS supporting memory re-mapping, allowing ram from peripherals, such as a video card (or video cards) to be mapped above the 4GB barrier. This is the easiest of the 2, as all BIOS'es now support memory re-mapping (I don't think anyone will be able to correct me on that point, but I won't get mad if you can prove me wrong there)
When both conditions are met, the 64 bit OS has all 4GB of system ram to play with.