OK, reading manuals sometime is very tedious. But after some great experiences i have to say that it always a practice that worth. Following the little guide about setting up enriched graphics with GRUB 2, the right compendium is a little refresh about setting console modes on Linux.
To discover which console modes are supported by our Linux Box all we have to do is to issue the command hwinfo --framebuffer, this will present us a list of supported modes, in hexadecimal values, that we can further pass to the kernel via the parameter vga=0x317 (HEX) directly, or the correspondent decimal value , in this case vga=791. I remember that if you are using GRUB 2 bootloader this method has been deprecated (follow the previous post).
When using VirtualBox otherwise, we need non-standard console modes to fit our monitor, for example i suggest a resolution of 1280x800 with a monitor of 1680x1050, cause some 4:3 resolutions may be uncomfortable. To let the resolution of 1280x800 appear on your virtualized Linux all we have to do, is to create a batch file or directly execute a specific action trough the MS-DOS command prompt via the VBoxManage.exe (on Windows) or the same on other platforms (i think is VBoxManage.sh for Linux, correct me). Be sure VBoxManage is in tour path or change to the directory containing it and write down: VBoxManage setextradata "Debian" "CustomVideoMode1" "1280x800x16", as you note Debian is my beloved Virtual Machine, you MUST replace with the correct Virtual Machine name, for example "Ubuntu 9.04 custom x64", everything not just Ubuntu or x64 or custom, it happened.
It's done, there's also a trick to have the same resolution also for the GDM welcome screen (no more dancing resizing). When in GNOME choose Display settings and set manually the resolution to 1280x800, by resizing the guest window, when the applet will tell you i you want to keep the resolution given, just keep, at the next restart you will be into the 1280x800 VirtualBox realm.