Something the poster might want to consider is an eMac. You can get them fully equipped with SuperDrive for $999. The eMac would be faster than an upgraded Powermac, unless you add a 1.2ghz or faster upgrade since the RAM and bus speeds of the eMac are faster.
If the poster is looking for a long-term solution in the desktop G4, he might be asking too much from it 🙂 🙂 You need to remember that the Powermac would only have a 100mhz bus and 2x AGP, while the eMac has a 133mhz bus and 4x AGP, and the new G5's have 10 times the bus speed of that Powermac with 1000mhz and 8x AGP, plus SATA built in.
If I was planning on buying a machine just for iApps, and didnt need HEAVY processing power (G5-grade), I would definitely get an eMac. They are Apple's best current value. Later on, when the eMac becomes too slow for iLife '06 or the user outgrows the machine, G5 desktops will be alot less expensive and will be more current that an old G4 desktop is now, seeing as how PCs were much ahead of the Mac at the time with CPU, RAM, AGP and bus speeds.
However, I completely agree with the above post concerning the do's and don'ts of upgrading an old G4 tower.