nearly the same problem, my screen was black and mouse arrow, and then no start.
I have an ATI chipset too. No way to fix it for me
At the end I gave up, and i will install again 10.5.1
A question; your installation DVD can mount all the hard disks? If I boot from the DVD and then go under Utilities\Disk utility, the 10.5.2 can mount only the partition that is interested by the installation. This doesn't happen with the 10.5.1 Install DVD.
Maybe there are some lost files in it.. I'm d\l a torrent version now. Let's see