That depends on alot of things.
MOST IMPORTANT. You want to make sure that your motherboard is compatable with what you are getting. If you get parts that arent compatiable its like driving a car with no wheels.
Graphics Card Support - Pci express, Pci expiressx16, AGP up to 8X. These are what your motherboard can support for example. A motherboard with PCI expressx16 will not support a graphics card that is AGP 4x/8x.