An ATI Radeon 9800 is, I believe, the best video card that was released for that Mac. There was an X800 released, but it requires the 8X AGP slot of the Power Mac G5, so won't work in any of the G4s. Good luck finding one for $150 or less, though.
A flashed PC nVidia card may be a good bet, too.
But, what all are you wanting to do with it? If you just want it for "2d" stuff, then your 7500 is perfectly acceptable. If you want it for Core Image stuff, then a Radeon 9600 or higher, or a GeForce 5200 or higher would be what you want.