The key factor is that each channel (color) needs the same total amount of memory arranged with the same sequence of DIMMS. So if they are all the same size, this automatically happens.
But to have 3GB running dual-channel, there needs to be 1.5GB in the green slots and 1.5GB in the purple slots. If you started out with 1GB (2 x 512M) in the green slots, then put 2GB in the purple, most boards can't operate in dual channel mode this way.
I'd suggest trying 2GB, with 1GB in the first green slot and the other in the first purple slot. This should activate dual channel mode, see if the full-screen glitches go away.
If they do, then you can try adding in the 512M DIMMS in the empty slots, if the glitches return, the old and new memory may have an incompatability.
At POST when the system hardware report is shown on the screen, the memory report may indicate whether it's running in single or dual channel mode. If not, a system analysis utility like Everest Free Edition
can tell you and show changes to memory bandwidth between different configurations.
If getting the memory operation correct doesn't solve this particular problem, then at least it is eliminated as a possible cause and someone might have ideas for other possibilities.
Edited by Platypus, 05 January 2009 - 08:39 PM.