There are many user friendly/ier Linux distributions for the new comer in Linux. Ubuntu, Mandriva, OpenSUSE, CentOS, etc. are very much point and click OSs and have a good friendly support in their forums and Mailing Lists to ask for help, in case you get stuck with some pesky device drivers. Avoid beta and even more alpha versions as these are for rather more experienced testers and developers.
As Kitz says there is a learning curve that you'll have to go through and these days such a learning curve is not particularly difficult. It is strongly recommended to try different Linux distributions from a LiveCD (all/most distros come with LiveCDs that you can boot and try out, or install from). When you find one that you prefer in terms of applications, look, feel and most importantly drivers - i.e. it boots without errors and recognises all your devices - then take the next step and install it as the second OS on your machine.
The best OS for gaming is MSWindows XP/Vista. This is because the majority of games are only developed for MSWindows - that's what the majority of the population use and therefore that's the biggest market to generate sales from. Besides games developers, video hardware and drivers manufacturers are also in bed with MSWindows for maximising sales. These guys don't always share the code for their drivers with Linux developers. Linux is slowly catching up, but right now it's not really a gamer's system.
Finally, plan ahead as you don't know if you will stick with Linux or remove it from your computer in the future. Depending on what the particular distro does with your MBR (most install Grub in the MBR of the first disk) you could run into problems when you uninstall Linux from your machine. WinXP won't boot. Two key commands that you will need to get you out of trouble and avoid a WinXP reinstall are
fixmbr and perhaps
fixboot. You can run these from a WinXP installation CD, or from BartsPE LiveCD to reinstall the MSWindows bootloader in the MBR of the disk and if needed (because you messed up when you installed Linux) to the restore the partition record of the WinXP partition.
Good luck!