did you use the correct version of the ati fglrx driver? Debian (woody,stable) comes with XFree86 4.1.x.
I had these lock-ups when I tried the the nonolk-patch for the fglrx.o-kernel module sources. So don't use them for XFree 4.1.x (you don't need to patch them anyway). Just apply the agpgart kernel-patch. Ignore the warnings and be sure to load agpgart.o before fglrx.o.