I think the SupremeFX X-Fi 2, has an actual X-Fi chip, whereas the SupremeFX just uses X-Fi software. The Maximus IV Gene, I would expect will have better onboard sound than a lot of other motherboards, but I doubt it will be as good as the Xonar D1/DX.
Sound cards are more than just a chip on a circuit board. There are many components that are there to produce better sound quality, that just don't exist on a motherboard.
Whether the SupremeFX X-Fi 2 will be good enough for you, might depend on what speakers and/or headphones you use. Also, only your own ears can decide.