AMD/Ati cards such as HD 6670 is pci-express 2.1x16
I heard that 2.1 cards don't work with old motherboard which is 1.0(the motherboard you currently have)
GT 440 is pci express 2.0x16 card which currently does not have any issue with old pci express 1.0x16 motherboard.
Make sure you get the GDDR5 version- http://www.newegg.com/Product/Product.aspx?Item=N82E16814121426
If you want smooth gameplay you may need to lower some settings or play at 1280x720.
Upgrade the power supply first just to be safe.