X server says No screens found

ATI and NVidia cards are the two most difficult to install in Linux, especially if you use Debian. To get the most out of your card, you need to use the ATI proprietary driver; if you have an x86 or AMD64/EMT64 (the 64-bit Intel), then you can find a driver installer on the ATI website:

http://ati.amd.com/support/driver.html

But first, if you want the system running X (at least with crappy 800x600 graphics), edit xorg.conf to use the "vga" driver.

Before you can install the proprietary driver, you will need a number of tools:

1. install the kernel header:
apt-get install linux-headers-$(uname -r)

2. Find out what version of compiler was used to build it:
grep gcc /lib/modules/$(uname -r)/build/.kernelvariables

For example, on my machine that gives me:
CC = $(if $(DEBIAN_KERNEL_USE_CCACHE),$(CCACHE)) $(CROSS_COMPILE)gcc-4.1

So I know that gcc-4.1 was used

3. find and install the compiler:
apt-cache search 'gcc-4.1' (use the version that you discovered above)
gcc-4.1 - The GNU C compiler

apt-get install gcc-4.1

4. install the Debian tools to make kernel work easier:
apt-get install kernel-package

5. If all went fine so far, it is probably OK to attempt to use that installer. If the installer is buggy, you will need to specify things like "header directory" - if the installer was written by competent people, then it will automatically check /lib/modules/$(uname -r)/build (which should be installed when you install the headers in (1) above). If your system has a second version of gcc installed which is different from the version used to build the kernel, you may need to do something like:
export CC=gcc-4.1 (use the appropriate version)
before you run the installer.

Have a look around with google and read a few more articles on how to install the ATI drivers on Linux, but first check the ATI site for links to 'how to' and other things people may have written up.