1440x1080. This is the highest 4:3 resolution you can get with a 1080p monitor!
This mod will make the game run in 1280x960 resolution. Not quite HD, but definitely better than 1024x768!
How to use: Extract to your Sims 1 install directory. **Back up the original sims.exe**. Create a shortcut (or add the game to steam) and add -w to the end of the location under Properties of the shortcut. This will launch 800x600 mode of the game, but it will actually be much larger!
This does have some glitches. The UI is a bit fudged, but not as badly as if you force the game into a resolution that isn't 4:3.
Check out image 3. See how the menu shows stuff wayyyy on the right side? That's because that part of the UI doesn't "refresh". So whatever was there stays there unless something else replaces it, if that makes sense. It's not a big deal in build/buy mode.
In live mode, you can read sim's motives perfectly. But when you click any other menu (job, inventory, etc) the screen doesn't "refresh" and things get layered on top of each other. So if you click Job and then Skills, it looks like both screens are open at the same time.
To fix this, simply click the buy mode icon before changing to another part of the menu. This makes the whole bar blue, so the menu you click shows up properly. It's confusing to read but it makes sense once you play. You just have to click to go to buy mode before you change to another sim menu.