As was explained by jggimi, traditionally on Unix systems the kernel supports the graphics card in VGA compatibility mode until the userland X11 DDX drivers have a chance to reprogram it.
There has however been a recent change in direction in the upstream Mesa/DRI/Xorg communities to move modesetting into the kernel, Linux supports this so most of the userland X drivers are no longer maintaining their userland mode setting code.. and not writing any for newer chip families at all.
People are slowly working on it, but until then, avoid buying the latest ATI cards.. stay with the older r300-r700 radeon families, they're more than adequate.
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