first of all make sure u dont have any grapichs drivers from an old gpu.
installation:
place ur new gpu in the top pci-e slot, and connect the 6 pin power supply to ur gpu( this cable :http://www.playtool.com/pages/psuconnectors/pcie6.jpg , the left one).
also, i checked on google, and ur gpu requires a 400 watt psu, while ur pc has a 300 one( unless u changed psu's)
meaning ur current one might not be strong enough to power ur gpu( it might be strong enough, since companies say their cards require more power then the card acttualy needs, just to cover their a$$)