> Are there fallback options for those that cannot or do not want to use compositing?
Probably OpenBox. Which is my current fallback when I get tired of Metacity's McHappyMeal-ness.
As far as hardware goes.. At the current time; All ATI cards since the R200-days should be 'ok' for handling compositing desktops. Similar for Nvidia, as long as they are supported by the proprietary drivers.
You won't be able to do it with the Intel 8xx series chipsets which some got sold with the Pentium-M era laptops. The 915 and 910 Intel chipsets can barely do it comfortably. But with modern UXA/GEM/setup anything 945g and newer should have zero problems managing a compositing desktop. The performance hit from running something like 'compiz' is negligable.. and as the drivers support for acceleration improves (like getting rid of needing software rendering for any part of EXA or whatnot) then performance should only get better.
I am running a Dell Mini-9 with 1.6ghz Atom and 1GB of RAM and it uses the 945g-era video chipset. It can run OpenGL composition quite well.. Well relatively well.
Mind you this is Fedora 11 beta with latest-and-greatest-everything. It took a while for Fedora 11 folks to get the drivers beaten into a good enough shape that performance is decent enough. It seems like the Ubuntu 9.04 folks missed the boat a bit with that one, but I am not sure.
The only problems I have are when watching large flash video. If you use UXA and then force Adobe's Flash to use OpenGL* then on the Mini-9 you can play Hulu.com videos well if the video is not maximized.
~]# cat /etc/adobe/mms.cfg
WindowlessDisable=true # a little bit more stable
OverrideGPUValidation=true # a nice performance boost