It would be a step up from motherboard sound, but not a big step. The SNR on the Creative sound card is better so you will get a little improvement in that respect. I would look at a ASUS Xonar unit that would fit your budget.
I personally did notice the difference from ASRock z77 extreme4 onboard audio, to a Creative labs Audigy SE (old sound card). Music sounded better in the latter (music at 320kbps) So I would recommend it, but I dont really think you will notice anything while gaming since your attention will probably be focused in shooting around!!