If your site is already active, I suggest looking at your Google Analytics reports.
You DO have Google Analytics setup right? =)
In GA, under
Visitors > Browser Capabilities > Screen Resolutions you should see the top 10 resolutions used on your site.
I usually look at the top 3 or so based upon visits to the site, and forget about iPhone and small device resolutions as if there is a high number you are better off targeting them separately.
Here is the list for my top 10 based upon visits for one site for the past few weeks.
Code:
1. 1280x800 12,082 20.37%
2. 1024x768 10,853 18.29%
3. 1440x900 7,542 12.71%
4. 1280x1024 7,489 12.62%
5. 1680x1050 5,250 8.85%
6. 1366x768 3,599 6.07%
7. 320x396 1,540 2.60%
8. 1920x1200 1,424 2.40%
9. 1152x864 1,422 2.40%
10. 1920x1080 1,297 2.19%
As you can see, for my particular site, 800x600 isn't even in the top 10. (It shows up at #1 with 1.21%) The 320x396 is a mobile device so I don't need to worry about that. So making my site with a minimum of 1024x768 resolution in mind will take care of 80%+ of my particular visitors. (I also was running Tapatalk for my iPhone/Android visitors, however, when I did poll my visitors they said they were also ok with zooming in/out. Waiting for the 4.x version to be available.)
And based upon other sites stats, I am pretty comfortable with the 1024x768 baseline. Of course, your site might vary and that is why I suggest looking at your Google Analytics reports. It is possible that your particular demographic does use widescreens more but in general 1024x768 is the norm.