640x480 is called resolution as well as 800x600, 1024x768, etc.
You set the resolution under the display settings of the control panel on your pc. Simply put...if I set the resolution to 640x480... then an image 640x480 would completely fill my screen.
That same image on an 800x600 would have space all around it.
I beleive there are 72 pixels to an inch if I remember correctly.
I set my screen to 1280x1024 which means I could fit 2 of those 640x480 images side by side...it would be very small though.
As a general rule I normally build to 550x400...this is because browsers have toolbars and status bars and menus that take up space at the top and bottom of the browser. This allows for the user to never have to scroll horizontally.
I think according to statistics the largest magority of users are set to 800x600 so you could probably be justified in building your site to this resolution.
In dreamweaver the pop-up feature allows you to set the size of the window that you want to pop-up. I use pop-up windows for guestbooks and such... I do not want to cover up the page that is already open so I will make it slighly smaller.
Tek-Tips has a pop-up window in the top left corner upon entry. This is a prime example.
Ya' Gotta Love It!
X-)