If you have some architectural skills, submit your visualizations of environments and locations. If you are an interface designer, some screenshots of your work would help. But when it comes right down to it, we need people who can draw characters and visualize worlds. The other stuff is just gravy. What we don't have much interest in seeing is fine art.
Games are definitely a commercial endeavor, featuring highly polished commercial art.
A sample or two of fine art is acceptable and sometimes helpful, but you need to show us you can do the commercial stuff too. The other half of the equation is your ability to visualize your work in both 2D and 3D computer media. Showing us your work in Photoshop or 3DS Max demonstrates that you have the basic technical skills need to create art for computer games. Include a disk containing samples of your computer work, and make sure it is free of viruses before you send it. Finally, if you really want the best shot, you need to show us you can animate in 3D. If you can take a character completely from pencil concept to a 3D animated, living, breathing entity that appears full of life, style and attitude, then you have the best chance to earn a spot on the team.