Depending on what kind of games you want to play. Some companies port their games to Linux like ID, and there is a small gaming community. If you are more hardcore into gaming I would suggest windows, but heed my advice before you take it into consideration of XP versus Vista.
Here go the Cons of Vista:
Compatibility - not everything that works in XP works in vista, however all new games and software will run fine in XP. We are no where near running a full 64 bit OS anywhere in any market. 32bit will still be around for several more years to come, possibly longer. Heck MS just finally pulled the plug on Win 98 like a year or two ago.
DX10 - thats right, DX10 is a pro and a con. The con side of it is, that since Vista is so resource hungry and offers no actual performance increase you will get less frames per a second in your games running DX 10. This may be fixed (I would hope so) in the near future or it may take some time. DX 10 only offers better eye candy. Google it, and you will see many examples of how DX10 runs games a lot slower than DX9 in XP.
Resource Hog - Vista is the most resource hungry OS ever created. Compare it to XP, Linux, and OS X (Unix, etc) and no OS requires that much to run. Other OSes also have just as much if not more Open GL eye candy and intuitiveness over Vista. I just hate to pay for something that does not offer me any kind of real world performance increase.
Security - its not more secure over XP, it uses the same permissions and the same kernel, the only difference is that it gives the illusion of more security constantly asking your opinion if you should open or download something.
SMB2 - increased network traffic by 1600% and breaks compatibility with other non windows platforms. Only affects you if you have a mac or a linux box on your network and wish to file/print share to a vista box.
I will stop there, but could go on and on.
Now for the Pros of Vista:
64bit support - break the 32bit memory barrier, which for some may be a plus. I don't know of any application that requires more than 3 gigs of RAM. Unused RAM is wasted RAM, but at the same time unused RAM is not always a super bad thing.
DX10 - looks pretty, gets the latest eye candy in gaming
Aero - if you want a 3D desktop environment and don't want to run Linux/Beryl/Compiz or OS X then this is for you
Some things overhauled - faster find and search options, stability monitor, it auto sets a lot of things up (which can be a con to some) for those who don't know how to network or do other mundane tasks.
XP Pros-
-Stable
-Runs all windows applicaitons current and future
-is not a resource hog
XP Cons
-only DX9 (until they release DX10 for it, there is an open source project)
-32bit (though there is a 64bit version of XP Pro)
-no nifty 3D desktop environment
Linux pros
-secure - NO known viruses in the wild
-open source - you can always find a solution for free for whatever you are trying to do
-many choices and flavors to choose from
-great online community support
Linux Cons-
-Higher learning curve
-It is not windows, many people expect it to be
-There is always a linux alternative, except for gaming
-some compatibility issues but not that many
So, really it depends on what you want to do. The choice is up to you and I just listed some facts to help you make your choice. My personal opinion is that vista does not offer any reason to upgrade at this point in time. This is my opinion and you can search through these forums to find all what I have said about vista if you truly want to know.