When making the decision on the minimum requirements for a web application on client machines:
To ensure the usability of the application is not compromised
W3Schools (for example) has some statistics on browser displays. As of January 2010:
Higher 1024x768 800x600 640x480 Unknown
76% 20% 1% 0% 3%
The Higher figure links to a new table. From the figures it looks like 1024x768 is the effective minimum screen resolution. However, you should remember that not everybody runs their browser in full screen mode.
There's also a Browser statistics page. The browsers are broken down by version (e.g Firefox). With browsers like Firefox and Chrome it does appear that the majority are using the latest (non beta) versions. I suspect that this is because they auto-update.
Other, more general, statistics sites are available and you should probably consult a range to get a better picture of browser usage and screen resolutions across the internet as a whole.
Instead of targeting a specific browser and resolution, I would start with the assumption that you want your site to usable by as wide a variety of users as possible. Try to make things adapt to different screen sizes and work in all browsers. That said, I think its safe to assume 1024x768 for desktop browsing.
A more global set of statistics indicate that 1024x768 is indeed the most popular resolution, however the distribution of users is very different. Also note that netbooks, they suppose, are helping 800x600 hang on.
In modern browsers you can target ranges with Media Queries. They're awesome. Assuming you don't care about IE – although IE9 promises support for them. Conditional comments can be used to send a lowest-common-denominator-resolution to IE, or JavaScript can be used to dynamically load stylesheets. Loading CSS through JavaScript dynamically is, in my opinion, an awful idea.
A web designer named Jon Hicks recently redesigned his website using them, go there and change the size of your browser window to see an example of how they work. There are layouts from 1- to 4-column, depending on your current maximum viewport width.
Media Queries allow you to not have to stress about things like "Will my site support the iPhone and iPad?" or "How can I target X device's screen size to maintain usability?". You just build designs for as many screen sizes as you want to support and the Media Queries will take care of the rest, depending on the user's browser when they get there.
You can base your decision of the recent statistic about the resolution usage. If you want generic number, you can find some from Net Applications and if you want precise data, you can use Google Analytic on your website for that.
The first thing you need to do is determine the target demographic who will be using your web app. For instance, if you are targeting a predominantly wealthy US/European audience (such as selling high-end products or a hi-tech magazine site) then you'll have quite different requirements than if, for instance, you are making a web-app that is aimed at older people, people in less developed countries etc. where old PCs and low resolutions still predominate. I know from experience, for instance, that many local government clients in the UK still (shamefully) use IE6 as their only browser, which could be a major factor if they are your client.
You then have to take into account that many people access websites on mobile devices - smartphones, netbooks, tablets and the like. These can have 'strange' resolutions or have a higher percentage of users of browsers such as Safari, Chrome and Opera Mobile. They can also have different accessibility requirements or may require different style-sheets.
One of the best ways is to use Google Analytics (perhaps from an existing site) to determine your actual audience. You can then get actual figures to see browser breakdowns and resolution breakdowns that will help you decide what is right for you.
Check the worldwide browser market share statistics
This question provides a comprehensive list of sources to look up worldwide browser market share.
Here are the top 3 2010 averages for StatOwl:
1024x768 26.75%
1280x800 18.69%
1280x1024 11.57%
Here are the top 3 2010 averages for Net Applications:
1024x768 24.27%
1280x800 17.98%
1280x1024 10.83%
According to the data, your resolution minimum should be 1024x768 but it also depends on your demographic.
If your site is geared toward a more corporate audience stick to 800x600. A lot of corporate networks are still running really old hardware with Windows 2000. If that's the user base you're targeting, using the 'current norm' of screen resolutions may alienate your audience.
http://isie6dead.com/ although this link was more dynamic a week or so ago, the point underlying its existence is still valid: there are way too many IE6 browsers out there and will continue to be probably for the next decade.
For my own personal projects, if you are using IE6, I don't care if my site causes monkeys to fly out of your posterior. For a job, the requirements may be different, and in general, the more public and older your target audience is, the more likely your site will see IE6 on 640x480x4.
This question cannot be accurately answered without a specified audience and completely ignores the burgeoning hordes of iPhony devices.
As mentioned in a older post, the Wikipedia article on browser market share consolidates information from 6 major sources and that stats is probably most effective in deciding which are the important browsers to target.
Also worth taking into account is the content of your site. If you're providing a tech support forum for people having trouble with video cards or monitors, for example, you probably want it usable at 640x480.
If many of your users are older or otherwise likely to have weaker eyesight, they might have high resolution screens, but with very large fonts, so you need to allow for that.
