My previous desktop was a Pentium 4 paired with Intel 915 motherboard from MSI and to be sure that you won't have compatibility issue running newer graphics card on an old motherboard I would recommend getting a PCI Express X16 version 2.0 instead of the newer 3.0.
I heard that PCI-Express 3.0 cards don't work on older 1.0 motherboard.
I recommend getting either this HD 6670 card which is 2.0
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102950
or a GTX 550 Ti which is also 2.0
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125410
I recommend going with the GTX 550 Ti since it is more powerful.
The HD 6670 does not require 6-pin power connector and should work running from the PCI Express bus power.
The GTX 550 Ti however requires 1x6 pin power connector for powering the graphics card.