How am I supposed to power this 4 port Sonnet USB card if the PowerMac G5 does not supply any molex power connectors?
It has molex connector for optical drive. You can use molex splitter(s).
There's another way: use SATA Y cable + SATA to molex adapter and power the card from SATA power connector (HDD). If you got only one HDD, SATA->molex adapter will be enough.
I mean, most USB 2.0 or firewire boards that are for mac need molex power.
I think that it's not exactly like you wrote. Additional power for card is required (or not) depending on what devices you want to use with card. All devices with their own power supply (scanners, printers etc) will work without molex. If you want to use bus powered devices (a lot of), card might need this molex. It must have something to do with overall PCIe power draw, which is limited to about 300W for all (four in your case) PCIe slots.