The way i think of it is this:
One card on its own will draw full frames by itself, as fast as it can send out with the Vertical Sync not applied.
Two cards will make that one card only draw half a frame by itself (making that powerful card only do half the work it did before, so all of its power can go into that half a frame). The other one can then focus on the bottom half, to finish the picture, to produce the frame about 2x faster (I say about, because its not always the case, depending on what each card has to draw on screen)
There are also other settings, like getting one card to do one whole frame, and the other to do the next whole frame so 2 are ready, but its the same principle.
WHAT THAT MEANS:
Well, you asked will it make graphics better. Well no, not in that sense. They'll only ever be as good as programmers intended, thugh, you might be able to put setting on higher than using one card, so you'll get better graphics and frame rates from doing that.
one 7800GTX is more than enough for todays games though, but in the next year or so, two will be more than welcome in todays games as graphics are shoved that little bit higher