There are options with nested X servers (Xnest or Xephyr) and the like, but you lose some stuff (3d acceleration). agdf did some work to make two X servers run on a single radeon, but those patches are out of date now.
One card per seat is what pretty much everyone uses. I got an extra PCI HD4350 for a silly amount of money -- even that's an overkill for casual browsing.
MAKE SURE to use similar gfx cards, preferably running the same driver. It might work with other combinations (Nvidia + AMD, Intel + AMD, etc), but this is rare, and why risk it?
- Is it easy to get running? Like, is this already in a state where I install something through my package manager and be done?
It's not hard if you follow a tutorial, like this one: http://sadevil.org/blog/2010/04/04/multiseat-on-debian/
- What happens to 3D acceleration in a multiseat configuration?
We are two at home, both adicted to the internets (so we spend a lot of time in front of our respective computers).
I have a PhenomII quad core and 4GB ram, and that's plenty. A dual core should be enough for most situations. Of course, the other seat gets a bit sluggish if I issue "emerge -udjv @world", an hog all the memory, but that can be planned around