As far as I know it's a little bit difficult to run Linux on laptops (problems with standby/suspend, problems with the graphical system,...). Have a look at e.g. http://tuxmobil.org/ to see wether your Dell laptop runs Linux smoothly ;-)

No, Linux won't break your system - although you might. ;)

Which system do you have?

Various Linux Kernels support SpeedStep Technology - the CPU throttling you mentioned.

As far as the warranty is concerned, I wouldn't think so, but I don't know.

I have been running Linux on my Inspiron 600m for over a year without problems. I had to utilize my warranty for a new AC adapter and had no problem.

I've tried Mandrake on my laptop and it worked OK, but after two weeks of only "OK" and switched to Gentoo and it's been lightning fast ever since. Only backside with Gentoo is that it takes a long time to install (be ready for at least 6h) and that its learning curve is steep for newcomers to Linux in general. It took me six weeks to get everything like I wanted it to be. But getting back to the point, when you have it up and running it's far better than Windows, since you can optimize it for your own computer. I've had some issues with the resolution though, I can't get it any different than 1400x1050 on my widescreen LCD monitor. But I'm working on it, it's no big deal though it looks great anyway.

See for yourself ;)
http://www.lynucs.org/index.php?screen_type=1&screen_id=8828835441bf58551d928&m=screen

Take a look at www.gentoo.org if you feel you're up to it, otherwise Mandrake will work fine as a standard desktop system.