The AGP slot on many Quicksilvers does not provide enough power for the ATI Radeon 9600 Pro PC/Mac 256 MB card. Your 64 MB card may not need as much power, or you may have a Quicksilver that provides a bit more power than most. The 256 MB is the only Mac 9600 card ATI is currently selling. I bought one three months ago for my Quicksilver (2001) and it sometimes worked, but usually would not start after sleep or power off. After a couple of months and two replacement cards, ATI admitted that the card would not work in some G4 models. After I bought the card I found others in these forums had had the same problems since September 2005, and there were many bad reviews of the card in the US Apple store. (I had not seen them before because I am in Canada, and the Canadian Apple store did not sell the card.) I sent copies of the reviews and discussions to ATI. The US Apple store no longer lists the card and ATI now has an article in their support web-site listing G4 models that are not supported with this card. (Search for 737-21888 in ATI's support section to read the article.) Of course they do not warn you in the sales section of the web-site.
ATI eventually replaced my card with a Radeon 9800 Mac Edition 256 MB. This uses a cooling fan, and has one single-link DVI and a VGA, instead of two DVI (one dual-link) ports, but it does work (it gets extra power via a cable to a disk drive power connector). It did occasionally leave small bars of video noise on the desktop. They looked like the video artifacts that others were getting when the over-clocked ATI cards, so I reduced the card's memory clock to 5% below the factory setting and have not seen any more video noise. I did not want to push my luck by trying to get ATI to exchange this card as ATI had been very reluctant to do the upgrade (no exchanges after 30 days, etc. even though I had filed the first complaint immediately, and it was ATI that took two months to admit there was no way to make the original card work in my machine.) I have seen reports of other video cards with memory that was not quite up to the speed specs.