i can receive from my work one of these: Dell D520 or Dell D820
same procesor, same memory, same HD
in D820 the screen 15.4 in D520 15
D820 weigh more
please advice me
My advice is get a D520 and an LCD monitor for the screen. My experience as that the more portable laptop is the more I like it, and the problem of a small screen is solver when you're at your desk by plugging in to a 19" LCD.
I like the D820.
One big thing to check is the lcd screen resolution. If your eyes are good, you will probably prefer the higher resolution lcd monitor, so that your windows can be bigger. If your eyes are bad, you'll just blow up the font size and lose most of the benefit, so save the money and get the lower resolution screen.
Also check for things like IEEE 1394 ports and whatnot. The D820 I used had one, and it made playing with MAC users a little easier.