I just put together a Shuttle XPC with built-in sound card (5.1 ch) which can't be removed. Luckily, I also just won a portable amp by Crosroads at the last meet

After pluggin git in to my PC setup, I noticed quite a bit of improvement on my ATH-A900s (more noticable on portable MP3 player setup, but I would not be able to amp at the gym anyways)..
I would like to get any suggestions on sub-$150 amps for my current setup. I can't upgrade my setup (only have 2 pci slots which are in use). Do I need anything else to improve the sound from my setup or just stick with the Crossroads portable amp?
Current Setup:
Generic 5.1 sound-card -->
Crossroads Portable Amp -->
Audio Technica ATH-A900 / Sony MDR-V6
Another question, if I am getting some standard
Creative T20 speakers, will they also benefit from an amp?