This may be old news, but I only came across it by accident so I thought I'd post a note here.
I read a review of a 10K RPM drive SCSI RAID setup on xlr8yourmac.com in which the reviewer mentioned he was using a Dell branded Adaptec 39160 dual channel Ultra3 SCSI card. He didn't say much about it other than he flashed it with Adaptec's current Mac firmware.
I picked up a PC 39160 off Ebay for under $50. I installed it in my Quicksilver, booted into 9.2 then downloaded and ran Adaptec's v1.2 firmware updater. Success! The card is now recognized in 9 and with a driver download, in X.3 as well.
320MB/s (2x 160MB/sec channels) for under $50. 
Now I need to decide whether I want 10K RPM drives in the G4 or if I should go external because of heat concerns.
-Tom
Tom;
Interesting. I think we hear so much about PC Video cards not being able to be flashed, we automatically assume that it means ALL cards. I have flashed both a Compaq OEM 2940UW and an Adaptec 29160 for use in my Macs, both times worked like a charm, and with FULL functionality. No "halfway" like some Radeon cards.
Here's to good ole MAC ingenuity!
FWIK Adaptek 2940s can be flashed, given a ROM upgrade. It's nice to know that newer cards don't need hardware mods to work.