Halo: Combat Evolved and Unreal Tournament 2004 aren't that demanding. Even my 6800 NU could run both at maximum settings and still get great framerates. The Elder Scrolls IV Oblivion on the other hand...now that game will definately benefit from a X1900 XTX.
When I put the settings on high for my X1900 XT for the first time, I fell in love with Oblivion. I've never seen such a pretty game in my life.