Linux Mint Forums

There should be something under Login window, but I don't find it now ....

Just curious, what is your current resolution set to now in Mint?

First, remember that the 'vga=791' entry in the kernel options in the menu.lst file affects only the initial screen resolution (the splash screen or the text displayed during the boot if in verbose mode). It does not influence the X server, and in fact you can get into some frustrating problems because the initial boot resolution I believe is determined in the 'initrd' [initial ram drive] file.
So your problem must be in the Xorg config file, more specifically in the X configuration.
If I understand you correctly, you would like the login screen to be at '1024x768' but it is going into '1280x1024'? So I assume your display is a CRT because LCD displays are always run at their native resolution (the highest they can do) because otherwise they look like crap. Of course CRTs have always been able to be run at one resolution level higher than you can stand to look at and use them at.
The normal home for the 'xorg.conf' file is in '/etc/X11'
However, having just installed Linux Mint for the first time (I now use Ubuntu/Kubuntu mainly on most of my machines but have about 12 years of linux experience behind me) I notice that at least on my setup the screen resolution isn't defined in 'xorg.conf'.
One thing to try is to go into the "KDE Control Center-Peripherals-Monitor and Display" and see what the setup defined there is.
Also check out 'KDE Control Center-System Administration-Login Manager' and check some of those settings.