I have tried Ubuntu 7.04, Fedora, PCLinuxOS.
PCLinuxOS works best out of the box, and looks and feels very windows-like. The best distro for ex-windows users.
Fedora not as user-friendly, but rock solid stability.
Ubuntu: stability issues, I dont like gnome.
It will be about 2 years before you can replace Windows with a Linux distro, if hardware vendors get on board (Dell already has). If you can't wait, the best Unix for home use is OS X, plus get console for games (Direct x 10 is worse in many ways than Direct X9 ever was)
Never had any problem with dual boot Linux and windows.
The thing about Linux though is the way folders are laid out and named eg there are some really important useful files in a directory called "etc" WTF. And it goes on.. eg fstab i mean there is no "libs" for libraries, startup-sequence or user-startup, unless you already know what these files are called.. it really is not all that user-friendly.
On the forums most Linux users often say stay away from 64 bit Linux as its unstable and often incompatible and not significantly faster.