This is a tough call, the upgrade for the Harmoni 600Mhz G3 can be found for about $300 (pricegrabber.com), but a refurbished eMac from Apple with a 700Mhz G4 can be had for $600 and it has a faster everything. I have played around with this idea alot, but after considering a faster HD and more ram it just falls apart. The only way this would be an OK investment is if you have already spent some money on your iMac and are just trying to extend it a little longer.