You could use a program like Alcohol 120% to make an image of the game's DVD and then you should be able to mount it also using Alcohol 120%. So, you could store the DVD images on your USB drive and use Alcohol 120% to mount the images. Alcohol 120% makes a virtual DVD drive on your computer that it uses to mount the images to. Your computer will see the images just as if they were actual DVDs in an actual DVD drive.
Unfortunately, Alcohol 120% costs $55. You can get it at
alcohol-soft.com. I'm sure there are other programs that will do the same thing for free, but Alcohol 120% is probably the most pain free way to do it.
Also, you can use
Steam. It's a game download service made by Valve that has a pretty large offering of games. There's also a community, friends list, etc to go along with it. And don't forget about the sales! Steam has a lot of sales on games with prices far below retail.