It also becomes very noticable in fast action games (like a traditional FPS Shooter) as opposed to an RPG where you tend to walk around instead of turboing around corners trying to kill stuff at all times. my rule is 50+ for FPS and 30+ for MMO/RPG
30 FPS is considered the "Magic number" by most reviewers. In truth it varies from game to game as already said. If your looking at review sites be aware that they use top end hardware, so unless you are as well you can expect less performance than what is reported in the charts. Personally i look for 40-50 in most games.