Just thought I'd put a plug in for a brand new amp that will be shipping before summer's out. the new x-head from av123. I've seen some frequency response charts and it looks like it'll be a solid unit at an entry level price. I've got one that should be on it's way to me soon and I'm hoping to be able to bring it to an upcomming head-fi meet so stay tuned for impressions.
bottom line, you can buy the amp after the headphones so don't rush into any decisions.
I would highly recommend the HD595's over the HD555's. I found them to be more resolved and detailed, slightly more satisfying. Both will benefit from a good amp. For games I feel the ATH A700's are a bit better, but they just aren't nearly as good as an all-around headphone if you plan to listen to music and movies, especially since you enjoy acoustic music (folk, jazz, classical, acoustic rock & pop)