Finally I got it!
My ATI Mobility X700 card is currently running with completely free driver “radeon” under a Debian Unstable. I spent a lot of time configuring it, because for some strange reasons the monitor often went blank screen, but in the end, without external or taken from CVS/SVN code, it works fine, even with 3D hardware acceleration. Relevant parts of my xorg.conf, plus some benchmarks and variuos considerations are available in a mail to debian-italian and in the subsequent thread.
Special thanks to the R300 project, the Debian X Strike Force and all Xorg guys!