For another sake of comparison, I took some screenshots from HL2: Episode One.
The pictures were taken on an ATI All-in-Wonder x1900. (The AIW x1900 card has the rough equivalent horsepower to x1800's, but with more pipelines). The photos were exported directly from the game and I made no alterations.
Below, the first shot was taken in 12x7 HD with all effects that the card allows on their highest recommended settings for the card. My LCD monitor doesn't support a full 1080, so I couldn't get "full" HD for a would-be PS3 comparison. So it's roughly 360's 720p.
For the second shot, if you want to fairly see what it looks like on a TV screen you have to blow it up to full screen. Settings are on low/medium.
1280x720 HD Effects on highest recommended settings for card.
640x480 SD Effects on low/medium settings.
And before anybody freaks out about "OMG low/medium settings for teh Wii you hat3r!" There's no chance the Wii has the same horsepower but just happens to lack HD... but I also did grabs of another scene, with details on high for both, anyway. Just in case you're one living in that fantasy.
1280x720 HD Effects on highest recommended settings for card.
640x480 SD Effects also on highest for card.
Draw your own conclusions, I was just curious myself to see what the difference would look like side-by-side.