Has anyone tried that? Currently i am planning to buy a second hand Xbox360 to play Saints Row 1, Gears of War and Halo series.

Is it possible to connect a mouse/kb to console and play the games that way?

To my knowledge it is not possible to do it with any old kb & mouse. There have been purpose made Xbox 360 mice though, and also small pads with a few keys (like a small piece of a keyboard). There are expensive though, and you still need to deal with some other annoying console issues, like the inability to turn off auto-aim (it really becomes quite obvious how much the games aim for you when you play a game where you can't turn it off)

Yes, but you have to buy an adapter for it to work on the 360. The prices can range from cheap if you find a deal ($15 or so) all the way up to $90+, and they take some work to configure them.

The one I have is the xfrag 360 that I got for $15 on ebay. You have to connect a wired 360 controller to it for the lockout chip to authorize with the system, then you can remap controls to your keyboard and mouse. I tried several mice and only one worked, so after searching I found out Logitech M100 series mice work and bought one for it.

The higher end ones like the xim3/edge should have less compatibility issues, but aren't worth it at $90+ for just playing a couple games.

Overall I'm happy with the xfrag, but it isn't as good as having a kb/m on a PC. You can't do quick turns as the mouse has to emulate an analog stick, so it takes some getting used to and adjusting the sensitivity to your liking. For the price though its "good enough" for the 360 fps games I have.