VGA - Video Graphics Array.
SVGA - Super VGA. Has much higher resolutions than the VGA. Must be accessed via a graphics driver of some sort, whereas VGA normally doesn't need one.
The dos versions of UFO and TFTD use a VGA mode (mode-13h) that comes standard with almost all video cards, whereas Apocalypse uses the SVGA, which isn't as widely supported.
A lot of new video cards don't seem to want to support some of the old SVGA modes anymore. Beats me why they don't want a bit of downward compatibility.
The only advice I can give is to update your video card drivers or look for a SVGA emulator. Or, look for an older video card, such as an s3 Virge or s3 Trio, or maybe even a Voodoo 2 Rush. Although replacing a video card just to play one game is a little silly if you ask me.
NKF, narrow minded fuddy duddy who refuses to let go of the past and will not accept anything newer than 1979.