[halfprep455]

Someone gave me an old powermac b&w g3 a few days ago. I had an old g4 400mhz CPU laying aroundand I was wondering if the g4 CPU will work in the b&w.

[firefly]

It depends what kind of socket the CPU you have is designed for. The B&W will take processors designed for the ZIF socket (a large square grid of pins) - looks like this:

http://eshop.macsales.com/images/Tech/beigeg3/5.jpg

[DrBoar]

If you want to have slightly more speed an other option is to overclock the G3! (Accelerate Your Mac! - Yosemite Info) Allmost all B&W OC 50 MHz, some even 100 MHz and very few 150 MHz. I have used old harddisk jumpers to set the clock speed
Cheap and safe. A G4 will have marginal speed gains compared to a G3 if it not altivec code

[P]

Originally Posted by DrBoar 

If you want to have slightly more speed an other option is to overclock the G3! (Accelerate Your Mac! - Yosemite Info) Allmost all B&W OC 50 MHz, some even 100 MHz and very few 150 MHz. I have used old harddisk jumpers to set the clock speed
Cheap and safe. A G4 will have marginal speed gains compared to a G3 if it not altivec code

The gain for a G4 over a G3 is 15% for mixed code, but larger for pure floating point code. If the G4 will fit, your best bet is to do both: install the G4 and then overclock. The G4 should run at 450 MHz at least.

EDIT: Note that Apple uses Altivec extensively throughout the OS (9 and X), including in many memory operations. It accelerates more things than you think.