[Ti] video ram

[thor]

hi guys !

I have an old Titanium G4 550, 15". Brilliant computer, although the 
harddisk
died a year ago (it was only 20 GB so an 80GB was welcome).

Yesterday the screen got all screwed with horizontal lines all over and 
a
triple image. I had a look here:

http://discussions.info.apple.com/WebX?128@@.68916e14

and found people with the same problem. It looks like this:
http://homepage.mac.com/lodesys/.Public/IMG_0965.jpg

I tried connecting an external monitor and it's all lines there as well.
(so it is not the cable to the screen)

Well, today I ran the hardware test on the computer (although having 
problems
with the screen) and it says:

Video RAM: Error detected
Error Code: disp/13/2

Ok, so it is the Video Ram.
Now my question is:

Where is the video ram? Can I change that myself?
Or do I need to get a new logic board?

I hope you can help me or point to some sources where I can figure it 
out myself.

Cheers
thor