I would recommend checking out the database over at
xlr8yourmac.com. I've tried two PCI IDE cards. The first one I tried was the VST/SmartDisk UltraTek 66 PCI card. The card worked great under 9, but could not be recognized under X. VST/SmartDisk initially stated they were working on providing support for the card under X, but recently announced they would not be supporting it.
This led me to purchase the Acard PCI IDE card from OWC. I've been using the card for about a month now and am quite pleased with it. It is supported under OS 9 and X , natively. The card is also upgradeable to support ATA 133 instead of ATA 100. In addition, I've found my read/write speeds to be pretty good.
I've heard good things about the Sonnet card, but chose the Acard one because I found a good deal on it. Can't say I regret the decision one bit