Leopard (10.5) requires a /G4/ of at least 867MHz. The
kernel is
 compiled against a G4 and will not boot on a G3. The check can be
 bypassed with an app called LeopardAssist or an OpenFirmware hack. 
Compiling against a G4 implies there is AltiVec (without a non-AltiVec
option) code in the kernel. To my knowledge the Leopard kernel does
not have such a dependency; where did you see this?
Not that I advise bothering with Leopard on a G3, mind you :)
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  Cameron Kaiser * Floodgap Systems * 
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floodgap.com
-- People are weird. -- Law & Order SVU ---------------------------------------