Dare I say that perhaps your GPU has reached the apex of it's capabilities? It's an AGP card at, what, 256MB? Also, unless I'm misunderstanding, 1GB of RAM doesn't do much to pick up any slack for the graphics card. Hopefully, your CPU is greater than a Pentium 4 and more on the dual/quad core rating, though that would be rare given an AGP slot for your graphics card.
ATI's All-In-Wonder X800XT came out in, was it 2005 or late 2004?
I'm afraid that all the tweaking in the world will not overcome this obstacle.