Looking for advice: Computer audio for music composition

Maybe a DAC could be all you need. Personally, I have 2 soundcards in my system, and I seem to have less and less use for them everyday.

Creative X-Fi Titanium: Has Toslink input so I can connect it to my PS3 without having to switch the audio source on my DAC... and that's all.

ASUS Xonar Essence STX: Originally bought this for the amplified headphone jack, but then switched to external dac/amp, so now I use it's Coaxial-out to my DAC, and the Line-In for anything that outputs RCA audio, which I often use (VCR, consoles, camcorders).

I bought both of those before really taking an interest in my sound system (aka going a tiny-bit insane). Now, since anything I listen to goes to my DAC in a digital-format, the only difference the soundcard makes is if it does any processing to the audio. But most games today use purely software sound-processing, so they don't need special hardware specs to get the "best" sound. There's plenty of options to decode/process sound from music/video applications, so that leaves us in my situation where the only purpose my sound-cards serve is their various holes that I stick my plugs into. 

I cannot tell a difference between USB->DAC and my STX's Coaxial->DAC. 

Also, you might not need an amplifier depending on the DAC you get. I have a modest TubeMagic D1 DAC (~$200) and it's a damn-fine amplifier, so I eventually removed my Little Dot MK III amp from my setup (still being enjoyed at my work desk though).

Just figure out what you want to do ahead of time. Do you plan on connecting more source devices to your sound system? Do you use any instruments that could be hooked up to your computer? Do you play games? Do you need surround-sound?