First and foremost I would like to point out that when you make a forum post, right below where you type your text is a little section:
The Fine Print:
And point number three states:
This is not a support forum. If you need product support use our Support Ticket System.
I understand that you are frustrated, but we easily get 50-100 forum posts in a day... and while it seems like 'we should be able to read through all of those in one day' we do have various other things to attend to. My apologies that you have not received a response from a staff member at this point. I do feel like 'The Fine Print:' section does tell you the best way to get a response.
A thread that may explain why your card is having issues is available here:
The last post by our Lead Games Developer, Stefan Dösinger states:
*) Radeon X1600 cards on Snow Leopard are broken. Either disable fbos(makes everything *slow*), or hope Apple fixes the regression. Leopard works.
Unfortunately I don't know if Tiger does work (my Tiger machine would have no chance of running L4Dead). The best I can suggest is to play with your settings a little. If you still have trouble, please do open a Support Ticket or send an email to firstname.lastname@example.org
(either of which will get a much faster review by Stefan and the rest of our Support Team)