If you plan on running your line out from your sound card into an amp, that is a major weak link. Until you change your path, it makes no sense to spend much money on an amp. Garbage in = garbage out.
A better route would be to get a USB DAC, like this one:
http://www.amazon.com/NuForce-Icon-uDAC-USB-headphone/dp/B002VN9CRG/ref=sr_1_12?ie=UTF8&s=electronics&qid=1279255651&sr=8-12
That way, you're not crippled by the crappy line out on your sound card. And, forget about the "virtual surround sound." Once you get a little better amplification with less background noise (like a usb DAC will do), you don't need it. Plus, you'll probably soon figure out that it degrades the sound more than it helps. A pure 2-channel signal can be marvelous.