Q: Can I put a MacBook processor and hard drive in a PowerMacG5?

No.

(122764)

But you can likely use the G5's KB & Mouse on the MacBook.

I have an older black MacBook 2007 era and use a Henge Dock when I want to connect it to my TV with a wireless keyboard/trackpad.

you would have to check to see if this is compatible with your particular model:

http://hengedocks.com/pages/vertical-macbook

an alternative is a stand, which would allow you to use the laptop display, in addition to another external display, on your desktop:

http://store.griffintechnology.com/elevator

PS- I have replaced the battery in my MacBook twice now, if it starts to swell up it can interfere with the trackpad button, causing it to register the button as being held all the time, which interferes with an external mouse as well.  you can find replacement batteries here, an Apple store may not have them in stock.

http://eshop.macsales.com/shop/Apple/Laptop/Batteries

to hook up the monitor, you will need one of the appropriate adapters, not sure what knid your MacBook uses, my older one has the miniDVI, they make adapters to go to standard VGA or DVI monitors, there were a few different video ports on the MacBooks, you can lookup the specs of yours at www.everymac.com if you are not sure what the plug type is.