The GT 240 is quite out of date and rather low end. If you're playing games from 2007 and earlier, then it might work fine. However, for today's games, it's too low end.
I would suggest at least a GeForce 560 Ti, if you can afford one. Knowing your budget, the kinds of games you want to play, and other such details (especially your PSU), we can help get you the best card for your money.