Thats something I was struggling with too. Up until now, all racing games were fine. I'm not talking about hardcore sims, I'm more of the arcade type - NFS MW, Most wanted, Dirt, Fuel, etc. I've played all my PC games with a keyboard, regardless of the type.
But I recently got NFS Shift 2. That changed everything. I can play the straight up racing stages just fine, but when it comes to drifting, good god, a keyboard is the worst possible controller to use. I swear, a mouse would've been easier. You just don't get the fine throttle control over game as would normally. Its either fully on or fully off. A controller on the other hand is the difference between night and day. Just yesterday, a friend brought over his PS3 controller to which my PC was hooked up onto.
All those drifting events suddenly became dead easy. When you have a controller, you control exactly how much of the throttle and steering you're applying. The same goes for Dirt 3, but to a much lesser degree.
Dirt 3, due to its far more aggressive assistance settings works pretty well with a keyboard. But its just no fun. The game would decelerate you pretty hard even when you have the gas key pressed down. turning off throttle and steering assists would make the game pretty hard to play - it made my spin out multiple times in each course as compared to the controller. With the controller, Dirt 3 was a joy to play.
My verdict/recommendation: Get a PC controller/gamepad - even if its a temporary el cheapo one. It helps IMMENSELY when playing racing and flying games. For FPS's and such, the keyboard and mouse is king.
Sorry, I don't play RPGs so I can't comment on those.