I am creating an app for many users with a wide variety of monitor sizes.

I have been restricting my forms to 640X390 pixels, to accommodate users who might be running at 640X480, which causes quite a challenge in form design, trying to avoid crowded-looking forms, but still getting as much as possible on the form.

I am looking for advice regarding the best size for  forms under these
circumstances.  If I could move up to 800X600, it would  make a world of difference, but I might be inconveniencing the poor little 640X480 user.  Is it OK to assume that anyone worthy of my app should be able to run at least 800X600?  I suspect that many of my prospective users will be running my app on laptops, but isn't it true that even laptops routinely are capable of 800X600?  I don't care about the extremely small laptops, like the new Toshiba Libretto.

How do the big boys deal with this?

Thanks,
Joel