The main windown is 800 px wide, but the client told me to change it to 1024, because he thinks that its a new standart. I think it will look worse, what do you think? What is the current standart for fixed window wide? Any ideas?
statistically (depends on the target audience etc.) approximately 80% of the users are using the 1024x768 and higher screen resolution (of course the number is continuously increasing). so, in my humble opinion, the 1024 is the way to go now, especially if you're looking out for the future of the site.
I would say the client is right in this case, no point for you to argue. and you CAN make it wider and it will look OK - of course it's not just scaling the layout to the wider resolution. make some things smaller, some things bigger - it will be just fine.
The statistics at w3schools for display resolutions shows a fairly clear trend towards 1024 and above, however they do point out that many users still have screens set at 800x600, which needs to be considered.