Hmm... it depends a lot about what games you intend to play. There's games which don't work as good with Windows 7 as XP, but that's also reversed. Thinking about the term "modern games" I see that your graphics card will limit your possibilites to play them if they were to require DirectX 11.
I think that if you play older games, Windows XP is the best choice, but then we go off your topic about modern games.
Win 7 is the best for gaming. For older games simply run them in XP compatibility mode. Sometimes the UAC feature may need adjusting to allow older games to install properly. Also dont forget to update DirectX to make sure Win 7 has all the DirectX 9.0 components it needs. For new games you will seriously need Win 7 to get the best out of them especially games that rely on multicore CPU's.