Should I use Windows Vista or Windows XP?

My father owns an orthodontics business, and there are a bunch of computers at his office. While I was looking through some of his old discs, I cam upon a Windows Vista installation disc, and I asked my dad if I could have it, and he said yes. I use Windows XP, and I do a moderate amount of gaming (Half Life 2, Quake 4, Portal, and some older games). I had a spare hard drive lying around, so I swapped my usual HD with the spare, and I installed Vista. So far, I like it. Should I keep using, and forget about XP, or re swap my usual HD and forget about Vista. I have a Pentium 4 3Ghz and 2GB of RAM. I also have a Radeon HD 4650 graphics card. I do a good amount of gaming, and plan to get some more computer intesive games.