I have a Dell XPS 400 with an ATI Radeon x600 256mb PCIe video card. I recently installed iATKOS v1.0r2 and was dismayed to find that I was stuck with 1024x768 resolution. I searched the forums hard, but didn't find anyone who had successfully got 1280x1024 resolution out of the x600. On a whim, I decided to try installing Callisto_b008. My results?

1280x1024 resolution!
Mouse artifacts when windows close/minimize
Card is recognized as a x600, but with only 128mb memory

Since I'm just playing around, and certainly not using the leo partition as an everyday OS, I'm fairly satisfied. Here's how I did it:

1) Download Callisto_b008 from here: http://omni.110mb.com/callisto/
2) Follow steps 1 - 7 in the Installation guide found in the archive
3) For step 8, go to About This Mac > More Info > Graphics/Displays and copy down your device ID (0x5b62). Replace 4150 under the IOPCIMatch key with the four characters you found next to device ID.
4) I didn't need to do step 9
5) Complete the remaining steps, reboot, and cross your fingers

Have fun