Sound cards are not as essential as they used to be, but they still serve a purpose. They're largely irrelevant if you use some sort of digital output, but if you're looking for clean analog inputs and outputs they can still be useful, since they tend to be a bit more isolated from the motherboard components than your traditional onboard sound solution. They aren't particularly useful for driving high impedance headphones (you still need an external amp for those), but gaming headsets generally don't fall into that category.
As far as true 5.1 vs simulated, having true surround always wins. These headphones are a bit on the strange side, as they channel audio from the top-mounted speakers into the earpads. I'd have to try them myself, but that seems like it would limit its frequency response by quite a bit (this typically makes the audio sound kinda thin and/or hollow, which is likely why they include a little subwoofer in each ear—to try and give the sound more life). The surround effect would probably be fine, though I suspect the center channel could potentially get drowned out by the front left/right channels (it looks like it's basically sharing a physical audio "channel" with the front left/right speakers rather than having its own route).
It looks like there's definitely going to be left/right channel bleed for both the front and rear speakers, but given that they seem to intend for these to simulate an open 5.1/7.1 surround setup that's probably intentional. Definitely not going to be able to get hard left/right panning out of these if they're designed the way I think they are.
Basically, I think it'll probably deliver a pretty convincing effect if you're purely looking for positional audio, but I have serious reservations about the audio quality. Keep in mind that you're getting a headset with seven speakers (five full-range channels and two subwoofers) and a mic for $75 and set your expectations accordingly.