Now, all seems working right, the PC starts by default at 1280x1024, and I finally can use the 640x480 resolution...
BUT, only ONE of the TWO available !! O_O
$ xrandr
Screen 0: minimum 320 x 200, current 1280 x 1024, maximum 4096 x 4096
DVI-I-1 connected 1280x1024+0+0 (normal left inverted...