I'm going to have to go against the advice of others here and say the HE-500 is fantastic for gaming and I think you'd do just fine with them. I do mostly first person shooters, and not just the classic ones like Call of Duty. Left 4 Dead 2 has a really good sound engine behind it and a good set of stereo headphones will tell you exactly where the attack is coming from. I have been accused of hacking on more than one occasion while gaming with these bad boys. I've noticed the same in Battlefield 3 and most lately in Borderlands 2.
The HE-400 might possibly be better, but I don't see how. The only headphones I might say were better were the Heir Audio 4.A's but I doubt it.
At the end of the day, buy whatever headphone you want. If it's a good headphone with a good soundstage it will probably rock for gaming. Good headphones tend to be good for a lot of things, in my experience.
HE500 and HE400 are both planar magnetic monsters, meaning they share many technical attributes very closely, the difference then mainly lies in how they are voiced (frequency response). In that regard HE400s emphasize mid/upper-treble and de-emphasize upper mids, which makes them sound more laid-back and shimmery than HE500s (which sound more even and forward). That difference in voicing would account for different perceptions in terms of ambiance, immersion, and subtle cues in games, esp first-person games.
Now as for which voicing WOULD actually be better for games, it depends on the definition of "better", which would vary from person-to-person and game-to-game.