After a bit of reading it seems that both cards should sound about the same, the d2x having a bit of advantage since apparently some of it's components are of a higher quality (and it has an optical-in port), but then it's not a native pci-e card and it lacks a headphone amp.

I have no problems with sound in D3 and skyrim, i'm one of the rare people not having (gamebreaking) problems with the phoebus in fact 😛 since i don't use protected audio or asio. But then the positionnal effects are broken (dolby ht is not detected as surround setup and xear surround headphone sounds like a joke) to the point that i am forced to use razer surround software (which is surprisingly good).

The D2X is a great card, i can't think of anything better on the market if you want 7.1 output (creative Z series are limited to 5.1), so i'd say keep it and fork some cash on a headphone amp (fiio e11/12 or a powered CMOY amps are quite cheap and effective) and a split cable.

The sennheiser PC350 are pretty good, i haven't seen any other headphones with an attached mic that can compare to them (maybe the pc360, if you want a wider soundstage and you don't care about noise isolation, both ways. You should consider those and since they're only 50 ohms you wouldn't need an amp).