A few extra sticks of RAM can let you give the gift of available system resources; an extra hard drive will ease the mind of the animator-turned-packrat who can't stand to delete a single iteration of a project.
Video cards make a nice gift as well; most associate video cards with an enhanced gaming experience, but the developers behind the animation in those games need those enhanced graphics just as much.
Buying hardware upgrades for someone else's computer can be a little tricky, though. If you can, at least find out the make and model of their computer, and research the specs to see what kind of available upgrade space it has (for instance, if I were to upgrade my Dell Dimension desktop PC, I'd look it up on Dell.com to get the specs for the kind of RAM or memory I should buy) in terms of slots and compatibility.
