I heard it before that you can take two computers and network their processors together to use them as one computer with two processors. Is it possible to network my iBook G4 1.33ghz 14 inch with my Powermac G3 B&W and use its video card to drive the video on my iBook or take some of the weight off the iBook video card.

Highly doubtful...

Not without writing the software yourself.
Xgrid can utilise the CPU power of other machines, but only for embarrassingly parallel tasks.

I'm no programmer anyone want to take this challenge up and write the software. It will be a historical landmark,\. It would be greatly appreciated. and one thing i wan to be in the credits for the concept of the program.

Can Xgrid take advantage of another computers complete system. as in the entire mother board processor and video card.

Not sure about that one. I would guess the answer is yes. If you have more than one Mac running Jaguar or later, install Xgrid and have a play.
One of the demo programs for it is a mandlebrot set. This breaks a full mandlebrot picture up into several parts and sends them out to attached Xgrid agents. Each agent computes a part each and when the controller receives the full set of parts, it assembles them to make a whole.

Xgrid is designed to be an API as well, so developers can add its functionality to their apps. If it were built into Maya for example, then each available agent could render frames at their own pace, with the controller distributing and collating the finished ones. If Maya makes use of the graphics card to do rendering, then all agents will do the same. Of course they would all need Maya installed (or at least some Maya compatible command line tools for rendering etc) on them.
The only way this gets done is with real time apps is called SLI. This creates a fast connection between two graphics cards in the same machine. No Macs have this yet. Though I suppose in theory, if you stuffed all 4 of the PCI slots in a B&W powerMac G3 with Radeon 9200s, you could write software to distribute frames between them without significant delay. Should then render graphics at frame rates nearly 4 times as high.
Doesn't strike me as a simple piece of software to write though.

http://www.apple.com/macosx/features/xgrid/

Xgrid will not help you render in realtime to play games. Nor will any form of distributed processing. Think about how fast your on board graphics are (The MBP for instance uses 16 lanes PCI Express for graphics while ethernet, airport and the Expresscard/34 use only one lane each). To use another machines video card or cpu, the data has to be divided between available agents, then transferred, then processed, then sent back and the final data reintegrated and displayed. This slows things down way to much for realtime. On the other hand, its great for lengthy tasks like rendering. I believe Compressor uses a similar system to speed up video encoding where processing nodes are available.