When running some applications such as supertuxkart 0.8 with a high resolution (1920x1080) and MSAA 6x the screen flashes, shows massive corruptions and gets very slow. Just changing MSAA to 4x the problem is no longer reproducible.
Tested on current git with:
r300: DRM version: 2.18.0, Name: ATI RV530, ID: 0x71c5, GB: 1, Z: 2
r300: GART size: 509 MB, VRAM size: 256 MB
r300: AA compression RAM: YES, Z compression RAM: YES, HiZ RAM: YES
This is probably related to memory bandwidth since MSAA compression is not currently implemented.