If its as simple as a power supply, it sounds like a cheap fix, but man, an eight-year old computer? You are running on well-outdated hardware and software, and for that alone I'd upgrade.
I don't know if you need to spec a MacBook out to $3k either - a spec'd Mac mini will blow the doors off your old G5 for closer to $1k.
Fwiw, I made the move from a G4 to a mini years ago as Apple was not doing enough for towers and I figured I'd gamble on the lower hardware capabilities. I do video rendering on occasion, but unless you are a serious video editor, a modern mini will suit you fine, even for heavy lifting in PhotoShop.
Plus, there are all the advances in the modern OS - automatic data backup, app integration and mobile integration, etc.
