Yeah but you will need a new power supply for a new video card for the special plugs they require for power. Most of them require 2 power connections
My Gainward 7800GS++ (7900GT) does.
I used a 380w PSU on it for a while, using an adaptor to get the required plugs, but I could no longer overclock my system as high as I used to and then the socket that connects to the mainboard eventually melted
replaced the mainbaord with a 2nd hand unit from ebay and board a Corsair HX-620 600w PSU. Awesome power supply, and near silent. Gave me all the extra plugs and connections I needed along with a modular design so you can unhook any plugs you don't need