The only thing you need to be concerned with when pairing a graphics card and motherboard is the slot type.
There are in order of release date/card power : A basic PCI card followed by AGP then PCI Express ( also written as PCIe or PCI-E ). There are several versions of each format but with a few exceptions these formats are backwards compatible. So a PCIe card that is version 2.0 will work fine in a motherboard that says it is PCIe version 1.0.
As shadow70379 stated shader model is NOT a factor.
Pixel Shader 3.0 has been implemented for several years now.
Any nVidia card from the 6600's on up, and ATI 1300 forward will support Pixel Shader 3.0
As pointed out already, you just have to make sure the card you decide to get matches the type of graphic slot you have on your motherboard, that is the only correlation to worry about. (i.e PCI, AGP, PCIe)