If I read things right, you plan to install 2 512 MB video cards.
If so, leave out 2 of those 1 GB memory modules if you plan to use a 32 bit operating system.
Why, there is a limit of 4 GB of address space for a 32 bit OS. The upper 750 MB is reserved for system resources. Your pair of high end video cards can use over 1 GB of that upper address space all by themselves. You might even end up with less than 2 GB of addressable RAM space.
The work-around for this is to install a 64 bit version of Windows.
As to motherboards, I agree with osmeg, it is much easier to match the hardware to the motherboard than it is to find a motherboard to match your hardware.
For the hardware you list, there will be only a few motherboards to choose from. Having 2 16X PCIe slots will be the first limit.
After you find your choices of motherboards, start thinking about what other resources you need.
PCI slots, 1X PCIe slots, IDE drives, Legacy ports, Gigibit LAN. You get the idea. Once you determine the rest of your hardware needs, you can get to a short list of motherboards that meet your needs.
Chas