SLI means I have an identical spare for my sig rig should something happen.
I have an old machine with an AGP slot that currently has a massively underclocked/undervolted 7950GT in it, but I could throw a 6800 Ultra, Geforce 2 MX 200, or Riva TNT2 32MB in it should something happen.
I have a Geforce 4 somewhere, but I haven't seen it in years.