I can send you pictures, but the same problem occurs for all the ones I've tried, so I don't think the problem is here. I have a 2006 Core 2 Duo iMac with a Radeon x1600 graphics card. As I said in the initial post, the problems appeared after 2.6 beta 2, so you should be able to track them in the changes between beta 2 and RC1
We ordered this series of ATI card to reproduce the bug here. We have an idea of solution, but it will need to introduce a kind of 'compatibility mode' in GPU option. You'll need to check it to make it work on these GPU series.
As I told Lionel by email for the private build I tested, it seems that the "compatibility mode" checkbox is inverted in 2.6.2 RC2 : I need to uncheck it (which should activate the new mode, not the compatibility mode) for the issues to be fixed.