It would appear that there is some problem with the glx module when using the vesa driver. I see three possible solutions:
1) disable the glx module via an xorg.conf file. Since you're using the vesa driver, glx is pretty much useless anyway. I don't know that this will prevent the crash, but I'd say it's likely to work.
2) Update xf86-video-ati, mesa, and the kernel to versions that support 2D (and even some 3D) acceleration. I'm not quite sure what's needed, but clearly what's in -current is too old.
3) Use fglrx from AMD. dijetlo linked to that driver in his post.