It really depends on the game.
If you are playing native Linux games like Half-Life 2, Left4Dead 2, Counter-Strike:Source, Brutal Legend, 0.A.D., Quake Wars, Doom 3, Quake 4, Defense Of The Ages 2, Portal... and a bunch of indie games... then maybe a boost in performance.
But if you are trying to play Windows games with PlayOnLinux and Wine you will probably sacrifice a few frames per second.
You can always dual-boot Windows and Linux to do some testing.
My current hardware doesn't seem to have many problems with PC games in PlayOnLinux. Click my Linux Games link in my signature and you'll see how many games I have tested.
Some run better than others and some have problems, but most games are working fine.
Native Linux games always seem to run faster and are easier to install.
Two things to think about before trying Linux and games.
- Make sure to use a modern Nvidia dedicated card
- Two distros seem to have the best gaming experience: Ubuntu & Mint