If you want the soundcard to actually power the subs, then you need an EXPENSIVE card, that's what kind you need.
If you just want to hook up two powered subs, you can probably use just about any soundcard with a subwoofer output and use a splitter, or a soundcard with any outputs and an amp that will split off the low frequencies to drive the subs.
I avoided the whole problem years ago by buying a 5 speaker system that accepts either 4 separate channel inputs, or true 5-channel dolby digital surround. After that, I just needed a soundcard that would output dolby digital on an spdif output plug. They pretty much all do that nowadays, output dolby digital on either coax or optical outputs. That's the easiest way, but it's not usually the cheapest since you need an amp that can decode digital surround input and speakers.
