One thing to consider is what your hardware specs are. Since you mention gaming and that you currently run WinXP, I'd assume you have at
least the equivalent of a Pentium III, 256-512+ MB of RAM and a decent graphics card.
As it's already been said that gaming is touch and go on Linux and other Unix flavors at best, although some vendors are beginning to distribute Linux binaries of their games, you will probably want to set your machine up to dual boot, keeping WinXP on one partition/drive and putting whatever OS you decide on another.
For me personally, I have a soft spot for Mandriva and Xandros. Everyone else's mileage may vary, but I had very good experiences with them. PC-BSD is also very accessible, and so far I have had to only manually configure one file on my system since installing it. That it includes KDE 3.4 makes the desktop quite accessible to the novice and pro alike.
Hopefully I haven't said anything to start a flamewar. I am merely speaking from my personal experiences in the UNIX world. As I said before, other people's mileage will vary.