Look into performance vs power consumption.
Last i looked the best way for a home user was actually offloading the processing to ati video cards, the 5xxx series (iirc) were a number of multiples better than a modern i7 range cpu. Also it was on the verge of dedicated bitcoin machines hitting market which would have a processing power many times higher than the ati setups. The dedicated asic machines are now out.
Basically. At this point with dedicated machines hitting the market the "difficulty" which is what payouts are based off of is increasing at an incredibly rapid rate based off the total processing power of all the machines on the network combined. The power consumption of a g4 machine vs its processing ability would never work out, even at the high current value of coins.
https://en.bitcoin.it/wiki/Mining_hardware_comparison
according to this the 1 g4 on the list has a hash rate of 1 mhash/s
an ati 5970 works at 700 mhash/s
an avalon asic is over 60,000 mhash/s
http://bitcoin.sipa.be
look at the spike in the first graph.
Laws of economics basically state that producers of a given good will appear to satiate demand at a given price point.
With the huge amount of press and spike in bitcoin value over the last couple months. The vast majority of "production" is going to be done by a small number of parties with deep pockets for equipment. Its hit the point where mining is far beyond basements/closets and old machines.