Quake III Arena
Quake III Arena is still one of the most important benchmarks - and likely will stay until another 3D shooter will surpass its popularity. The 16 bit run is very theoretical, since the frame rates are of course fast enough for smooth game play even at 1280x1024. However, it is also the best way to show the real world difference between graphics cards.
Basically, your desired resolution should run at 30 fps or faster. A demand which even the standard GeForce2 MX is able to fulfil. I did not include the results at 1600x1200 but all three cards drop below those 30 fps than. After checking those Quake III results I cannot see any reason why I should spend more money for a MX400 board. The standard MX will do it.