So I was playing Half-Life 2 on my PC, 1152x864, everything maxed out, and besides texture detail (which is pretty bad now) it still looks amazing.
I know Half-Life 2 was released for the original Xbox, so I was curious to how it would look on it, and consequently, how it would look on the Wii (as they are similar in power). I turned down the resolution to 720x576, most things on low/medium and turned HDR to bloom. This is the result:
(right click and view image for full image)

Conclusion: it should be able to run on the Wii at a solid 30 fps no problem. Episode 1 and Portal would also work. Hell, even Team Fortress should be easy to put onto the Wii. So where's Orange Box for Wii, gentlemen?













