DualHead on Ati cards can be acheived via two means, you can use X's inbuilt Xinerama extensions or you can use the ati driver's MergedFB which provides pseudo-xinerama behaviour
Important things to note with ati cards is that if you use Xinerama you get no acceleration on the second head but it does allow for running the displays at different resolutions easily and reliably
At the moment I will just post a couple of configs that i know have worked for me in ubuntu with an Ati Radeon 9800se and a few notes, please note these settings are for two crt monitors (one on a dvi-vga adaptor), LCD displays may need different settings
This is a Xinerama based xorg.conf that provides 1280x1024 on one head and 1024x768 on the other, both at 75Hz refresh XorgConfAtiXinerama? (sorry lost this one atm, someone add if you have one)
This is a MergedFB based xorg.conf that provides 1024x768 on both heads at up to 85Hz refresh dependent on what the displays support XorgConfAtiMergedfb (modified from a sample found on the net somewhere, can't remember where sorry)
included in the display section of your config the card will use the highest refresh the displays support instead of the configured refresh rate, handy if you have 1 display that has a low refresh thats bearable but you dont want it that low on your primary display ie. card can use different refresh rates on each head
Option "MergedXineramaCRT2IsScreen0" "on"
Swaps the primary and secondary displays for xinerama (apparently some programs default to one or the other)