Graphical superiority isn't requisite for me to consider a game to constitute "art", but it definitely helps. Storyline, innovation, depth, these are the things that make a game art. Games that try to break out of the mold and do something new and innovative usually constitute art, as do games with an epic and genuinely engaging and interesting storyline (usually happens with games that break from normal storytelling conventions as aside to sticking to the old, tried, and true).

This isn't always the case though, and it's really subjective. I'd say a good game speaks for itself on whether or not it could be considered art.