PCI-X is simply just double clocked standard PCI. The FirmTek "PCI-X" cards simply mean support for that extra bandwidth feature but they are still fully compatible in a G4 tower.
FirmTek bungled it a bit by just flat out calling it PCI-X. It should be listed as PCI-X support. Just look at any of their non-PCI express cards. All support both G5 and G4 towers.
I have one of these very cards (FirmTek 1S2) in my G4 and it is listed as PCI-X but also lists support for G4's in the requirements.
To the original poster..
I strongly recommend the
1V4 from FirmTek as it allows 4 drives so you can use all 4 drive bays in the MDD's. If you only ever want up to 2 drives or need to spend less you can get the 2 channel
1S2 I have.