There is no X1950XT or XTX AGP, only the X1950 Pro AGP. But on your system, there would be a huge difference going from X800XT to X1950 pro, depending on the game and settings you play at. http://www.tomshardware.com/2007/02/01/agp-platform-analysis/
Look at Tom's review (linked above) as the 3400+ is a perfect comparison for your system. The X800XT is a bit behind the 7600GT/7800GS overall(the higher clocked x800xtpe would be about equal), but close enough and it would take some games. So just use those two for a comparison and you will see in GPU intensive games the X1950 pro spanks those other cards. You'll also be getting a capable SM3 card that can do HDR in Oblivion and Farcry and is also capable of playing any SM3.0 only titles that may appear like SC:Double Agent.