Unless you are playing games meant for a very young audience virtually all games contain immorality. Language, sex, and morals conversely opposite of the Christian faith are a large part of all media. We really can't expect anything different when most games are created from a secular, humanistic, Taoist or Shinto-Buddhist view points. However there is no reason to single out games from any other media be it movies or the internet. If you want to determine whether you should be involved with a specific piece of media I'd ask myself the following things for starters...
Ask God about it (pray). It's not only asking if it's something you should be putting before your eyes but asking if this is something God would want me to spend his money on supporting. Just because you can avoid looking at a certain part of a game doesn't mean anything to developers as you are still buying the game. Our purchases shape what type of media is going to be made in the future and by that shape people.
What does the Bible say about it's content?
Will it cause me to sin? I've had problems with hearing a cuss word repeated incessantly and because of it I find those words coming to mind when I am searching for something to say sometimes. In the case of sex if you are buying a game knowing it will cause you to lust after a woman (even virtual) in your heart than it's a sin.
Ask follow Christians their opinions (hey you've got that covered

). Of course what God says directly is always much more important but it's good to ask others in case you've overlooked something. It's one thing to ask strangers though and another to ask people who are close to you. If you can talk to people like your mother and father about a piece of media without fear/shame it may be a good sign it's ok.
Nothing of this world is perfect don't expect to find any media without flaws. Ask not only what is negative about a game but what am I getting that is positive reinforcement. In my opinion of Team Fortress 2 it does have a few words that I deem inappropriate and as it is an online game I've been exposed to some users' perverse content (going online is like going out in the world :/). On the positive side I find the violence non-realistic and not in an immoral context (I'm not playing a criminal/bad guy). I get to game with a group of Christians which I cannot do with some online games as many do not have substantial Christian groups. In addition the game reinforces teamwork and sacrifice to win more so than many other multiplayer games.
Having said that be aware that the general mindset in today's culture is to be all inclusive. Allowing everything good and bad in a game is not acceptable because even if it has positive content that does not mean it negates the bad. The paradox is if sin is shown to have an agreeable outcome sin becomes just as attractive to people as good (or more so because it's often easier). However if that sin does not have an agreeable outcome who would want to play that path, thus what is the use putting it in a game? As Christians we should be working towards being better than we are so our money should go towards games that encourage people likewise.
In final if you are having doubts about any game I'd say avoid it. It is just a game after all and the day we can't live without a piece of entertainment is the day we need to step back and look at our priorities.
(Note: CGA is a Christian "Gaming" forum so don't think you are getting an unbiased opinion. It's kind of like walking into a saloon and asking if the people there think liquor is ok

. To be fair you may want to ask some non-gaming Christians their opinions in addition. In the end the choice is between you and God not anyone else though.)
(Another Note: If you play x-box you may want to check out Toj's
www.toj.cc X-box group and their forum section
http://www.cgalliance.org/forums/forumdisplay.php?f=69 .)