I know you have a limited budget but I would seriously look into the Mac Mini
I had not paid too much attention to them, but recently replaced an older admin machine (G5 tower) with the newest Mac Mini, and was quite impressed. We got the cheaper model and maxed out the RAM to 4G and I have to say its quite a little powerhouse! Fast dual processor, NVidia graphics, very quiet, purrs and runs stuff fast. I think replacing your G4 will see definite improvements in video playback and render speed, and the Mini is inexpensive enough to perhaps fit in your budget.
Franklin
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