Hello there, welcome to Head-Fi
Okay, so you want to get everything out of the HD 595 when playing shooter games, I assume you're talking about the experience itself, to get more immersion and feel from the games. Meaning that you're not talking about getting the full pro-gamer first-strike use out of the headphones, hearing all important sounds as soon and as clearly as possible, like footsteps etc.
What kind of a soundcard are you using now? The HD 595 have an impedance of 50 Ohms, meaning that they're pretty easily driven with a good source alone, so it's possible that a good soundcard is all you need, if your main activity with the headphones is playing games.
Sure they benefit from a good amp too, but buying a good amp for the Sennheisers is something you should do for music. FPS games rarely have music and you forget about the nyances of soundquality when you're in the middle of gunfire anyway
If you're using a soundcard that's integrated into your motherboard or it's one of those "wannabe-hifi" "HD-audio-whatever" bundled cards, you can start looking for a serious soundcard, like the Creative Sound Blaster X-Fi XtremeGamer etc.
Then your question about the sound setting, some soundcards actually have pretty nice features for virtual surround simulation, but you have to try those out by yourself, different games give different results with different soundcards
..oh and the "crystallizer" etc. features are usually bad, that's my experience with them, I've tried a few of the creative cards and a few external ones with similar features, they just artificially try to boost the audio, making always somekinda distortions on the way.