Save you money. The integrated on the motherboard is good and you wont notice any difference going with a new card. And if you did its most likely placebo effect doing its thing :)

I would try to keep it as simple as possible. PC > Focusrite Scarlett 2i4 > AT2035 for input and for output, use your current setup as is. But, I dont have experience using a setup like that so I dont know if it will be the best. 

Any reason why you want a dedicated sound card? Asus usually does a good job with their integrated audio and I dont think those A700x's are hard to drive. Same with the HD598's. Those will work just fine using integrated audio off the motherboard. Are you experiencing problems with your current...

  Unfortunately I cant. My experience is limited to onboard audio, various creative cards, and Astro MixAmp. And I've only used the ModMic version 1. Sorry :(

Yeah, from what i've read about the 4.0, it has very low volume :(. Im still using the 1.0 modmic and it works fine out the box with all the sound cards (dedicated and onboard) I've used. Only thing I can think of is, try using your onboard mic input and see if that makes a difference. At the...

Do you hear any hissing, crackling, pops, or hums using onboard? Or do you feel like your gaming experience is lacking?

What Im trying to say is that with your headphone, they dont need an external amp and the dac on the motherboard should provide a clean audio signal. But not all motherboards have good implemented onboard audio. Do you hear any hissing, crackling, pops, or hums using onboard? If the answer is no...

My experience has been: with low ohm headphones, power doesnt make a difference. Now this isnt true for every headphone and I havent listened to the psb ones that you have. And Im just going on what Ive used. But Im confident that you dont need a dedicated amp. And from a sound quality...

Those headphones are rated at 32 Ohms. I dont think you'll need an amp. An Asus or Creative card should be more than enough to power those psb headphones. Or just use your onboard sound if you dont want surround sounds (I find stereo to be better anyway but, thats just me). Get a soundcard if...

Modmic, hands down

Modmic is badass and voice quality is very good. If you go separate mic (highly recommended) then look at this thread for good headphones in your price range http://www.head-fi.org/t/534479/mad-lust-envys-headphone-gaming-guide-updated-5-23-2013-mrspeakers-mad-dog-v-3-2-reviewed

kind of a waste of cash imo. you'll be better of upgrading other things, like room acoustics, speakers, music collection. those things will actually make a difference 

whats wrong with what you have now? is it causing problems of some kind?

whats wrong with your ST? Seems like its doing the job just fine

Wow, you dont mess around lol. Let us know how you like that setup :)