I have an AlchemyTV inside a B&W G3 and another one inside a MDD G4, both work great with Tiger. There is a difference between both computers if you use the Alchemy to record TV shows, with the G3 due to the CPU power you must do some work with the codec and resolution to use to get a video without dropping frames, with the G4 there is not such issue, this G4 is a 1,25 GHz model but with a G4 500 MHz it worked great, your CPU power is enough.
As example, I was able to record at a 'personalized' 500*382 resolution Motion JPEG B with the G3 350 MHz and it worked flawless, no drop frames at all.