If you have a system built with the still quite good Nvidia GeForce 8800GTS graphics card, and decide to upgrade by adding a second one and running SLI, beware the Nvidia Fail.
Nvidia in their wisdom made an upgraded version of the card called the 8800GTS 512. It is not SLI compatible with the original 8800GTS, and the odds are you won’t know it until you’ve bought one and spent an afternoon wondering why SLI won’t work.
God damn you Nvidia.