If you want to set a custom resolution, which in my case was 1680 x 1050, all you have to do is go to C:\Documents and Settings\*ACCOUNT USER'S NAME*\Application Data\Atari\RCT3 and double click on Options.txt. Change the resolution value to your desired settings. Save and close the file.
For example, for me it was: C:\Documents and Settings\Eric\Application Data\Atari\RCT3\Options.txt
I decided to make the options.txt read only, but that's up to you.
Here's how my options.txt looks after changing the resolution:
I have been trying to figure out a way to run RCT3 at 1920x1200 on my 24inch widescreen monitor. I found the older custom resolution thread that specified basically the same method as you have described. I am able to navigate to the RCT3 folder in My Documents (I dont see the Application Data folder or anything, the RCT3 folder is in My Documents. However, there is no Options.txt file. I have tried selecting Show Hidden Folders/Files under Tools>Options. Nothing changed.
I then tried navigating to my Games folder where the main RCT3 files are located. There is an options.txt file in the main Rollercoaster Tycoon 3 folder. However, when I open the file, it has two short lines of text and NO mention of resolution or anything else you have posted above.
I have RCT3 as well as the two expansion packs. I try running the updater and it says everything is up to date. Have I missed it or is there something I can do to make this work? Please Help!!!
Haha... ok, so I see now that it was under Documents and Settings... I was able to find the CORRECT options.txt file! Of course I found it AFTER I posted. I am going to try fiddling with this and post my results! Thanks again for this info!