-Support 1600x1200--not just by enabling the video mode, but also making sure the UI and in-game text remains legible.
-Allow people to enable high-res textures. The UV mapping stays the same, so it's just a matter of including the high-res set.
-Enable a couple of effects using pixels shaders. For example, using pixel shaders, you can easily make grass look prickly and realistic instead of like wallpaper. This would have great effect on the single player mode where there are cut scenes and the camera is close to the action. Another example is the use of reflection maps, which would make shields and armor look shiny and metallic.
-This is more work, but creating level-of-detail (LOD) models would allow faster machines to use high poly versions of the character models. Creating LOD versions of some of the major MUs and infantry would give the game a big visual boost. You would especially notice it when the camera is closer to the action.
-Enable voice communication. This is something that was added to a Counter-Strike patch a few months ago, and in multiplayer games it makes a HUGE difference. Being able to communicate with your teammates just by talking can radically improve the immersiveness and strategic elements of a game. In fact, this one doesn't even require a high-end machine.
Given the complexity and long development schedule for a game like AoM, I wouldn't expect to see a sequel for a couple years--minimum. By then, a P4-5000 will be a good but common PC. And graphics cards will be able to render scenes several times more complex than what they can do now. For example, John Carmack recently said in an interview that Quake 3 rendered about 10,000 polys per frame, whereas the new DOOM will render 150,000 polys per frame.
Spending some effort to enable users to max out current high-end machines can add a lot of shelf life to a product. It also will obviously demo better and will get better graphics scores in reviews. I also suspect that the hardcore players of AoM who will focus on multiplayer will, in general, have higher end systems. Hooking them up just creates a more fanatical core fan base who will evangelize the game even more. I'm looking at the Warcraft 3 screens coming out now, and while it looks nice, it's very obvious the texture resolutions and low poly counts of the characters and buildings were geared around mid-range machines. That's fine, but it seems Blizzard didn't do much to allow faster machines to take advantage of more detailed, rich artwork and rendering effects. If ES does, it will give AoM a distinct competitive advantage.
Just my 2 cents. Sorry for the long post, but this issue is really important to me and I think many other gamers. Yes, it's possible to have a great gameplay experience playing pong, but having killer graphics makes everything sooo much better!