No need to purchase any new, major hardware to do this. Pretty much every Mac that has a tower form-factor since 2002 can run dual-monitors off of the factory-supplied graphics card.
Your G5 has one of the following graphics cards installed in it:
GeForce 6800 Ultra, GeForce 6600/7800, Quadro FX 4500, or Radeon 9600/9650/X850
All of them support running dual-monitors out-of-the-box. Depending on your combination of DVI and VGA monitors and depending on whether the graphics cards have dual-DVI or DVI-and-VGA ports, you may need a DVI-to-VGA adaptor (or vice-versa).