The Radeon 9800 is faster then the Geforce4Ti in OSX, but adds nothing if you wish to BOOT in os9, (helps in Classic). The new 9800 has an extra input from the power supply and will work, the 9600 works without the extra power input.
Go to OWC and you can get teh updated 256 MB version of the 9800 which will run perfectly in the Quicksilver, or the less expensive 9600 (dual PC/MAC). They also have some converted OEM stuff hacked to work in 4x agp slots, but if I was going to go for it I'd probably spend the extra for a new 96 or 9800.
Good luck.