Nope, this is only for headphone use. There's a version for powered monitors.    In this case, you need a unit dedicated to power speakers. 

I'm organizing my library after upgrading my HDD. Currently have 800 albums in Mp3 320 and 300 on FLAC

  Even better than the newest version of it (the 400i) in my opinion. You're good to go. :)

How about staying with Beyer and get the DT770 pro 80?

Pompeii 7/10 If you love drama and history it will suit you. Cool 3D effects and good acting..but i was expecting more. 

A music Buff? I'll try again 

Only with music unfortunately. They were at a friend house and spent some time with the portable rig listening music (setting 3). Overall a good experience.    May need to try them for a weekend on games and report back. 

And i still have the FX6300 

What about this? http://www.amazon.com/Replacement-Leatherette-Universal-Sennheiser-Beyerdynamic/dp/B00862522A/

Xiaomi pistons

Playing- Titan quest: Immortal throne. 9/10 Seven years ago i played the prequel and it was very addictive! 

The HE400 deliver some good bass, they will work nicely with your O2 combo. 

Welcome to head-fi!    My personal recommendation:  The Soloist SL. Suited for low/high impedance headphones, is able to power your Sennhs and AKg with ease. Does a fine job with some planars as well. If on the US it can be found for $550-600. 

  It's a 5W speaker, unless there's a new 10W version? :D

  It depends on the headphone sensitivity. The more sensitive the easier to drive.  For this model, a few guys recommend some extra power.    You mentioned the uses, but not your music genres. Please give us more info. 

The only option is to DIY.    A member used a HD650 headband and modded it to fit the HD565 which are pretty similar to the 265's.  http://www.head-fi.org/t/518202/sennheiser-hd565-appreciation-and-service-thread#post_7364416

As for the gaming part, they are not as accurate like other headphone models such as the AD900x or HD598 but they are still good. If on FPS you can buy a Mixamp and they improve nicely.     Nice recommendation by the guys above. 

  Op, what kind of sound and music genres you like? the S500 are fun sounding while the 400's are more revealing. Choose depending on that. 

  You can forget about the good mouse and keyboard for now, and spend on the most important components.    If in the US tigerdirect has AMD barebone kits that fit the bill. 

Orpheus HE90 anyone? 

Hello and welcome.   To answer your questions:   1- The STX II will have no problems driving the 250ohm beyers. I'ts a very capable soundcard.  2- I don't see the need for a separate headphone amp, unless you decide to buy hard to drive headphones. It will not affect the sound in any way...

I'd go for the Beyers, given their flexibility. 

Playing Black ops 2 for the second time, this song is amazing (specially if you are using the right headphones)   

Lazyboy, you need to state a budget for your build, i guess you want advice for the main components. 

If you need accuracy they are not recommended. For non-Fps (racing games, etc) they are good :)