nb1, I don't think you'll be able to install radeonhd driver without installing some packages from testing or backports.
Enable the testing repositories and configure pinning to not update your whole system.
Here goes a /etc/apt/preferences file that I use in a server which runs qemulator(only available in testing)
Code:
Package: qemu
Pin: release a=etch-backports
Pin-Priority: 1001
Package: bochsbios
Pin: release a=etch-backports
Pin-Priority: 1001
Package: qemulator
Pin: release o=Debian,a=testing
Pin-Priority: 1001
Package: libgtk2-ruby
Pin: release o=Debian,a=testing
Pin-Priority: 1001
Package: libgtk2-ruby1.8
Pin: release o=Debian,a=testing
Pin-Priority: 1001
Package: *
Pin: release o=Debian,a=etch
Pin-Priority: 700
Package: *
Pin: release o=Debian,a=stable
Pin-Priority: 700
package: *
Pin: origin ftp.debian-unofficial.org
Pin-Priority: 550
Package: *
Pin: origin www.debian-multimedia.org
Pin-Priority: 550
Package: *
Pin: release o=Debian,a=oldstable
Pin-Priority: 500
Package: *
Pin: release o=Debian,a=sarge
Pin-Priority: 500
Package: *
Pin: release o=Debian,a=etch-backports
Pin-Priority: -1
Package: *
Pin: release o=Debian,a=testing
Pin-Priority: -1
Requires some manual work at first but later works fine.
Or you might consider using 64bits lenny(if your cpu supports it). It has already entered the frozen state. Its release is expected this year, so it's with a good stability already.