Well, DX10 isnt going to work with XP iirc, so you'll need vista for gaming. Best bet is dual booting. Use Vista when you game, a linux flavor when you work. Ubuntu should be the easiest to start with, ubuntuforums.org is a great starting resource. You could even dual boot vista and XP if you are happy with XP.
Don't expect Linux to be as easy as using Vista. After all, you used XP for a while and Vista was just a small shift, not a paradigm change. Linux is very different and you'll feel like a fish out of water. Stick with it.
I don't see the need to pirate software, but its possible to put MacOS (which I recommend highly) on normal i86 hardware. You'll miss support and optimizations, but people often use it to decide if they want to move over.
There is no perfect OS. Find the one which inconveniences you least.
For the braver people out there, try OS/2, Plan 9 or BeOS(Zeta). Get extra geek points for using an OS that most people have never even heard of, let alone used.
If I had to tell you what to do: triple boot. 20gb XP 20gb Linux, rest vista for games. Try linux, if you get frustrated you can fall back on xp.