The top 10 reviews look dated even though it says 2013.
For gaming the Creative "Z" series are probably the best but since you just have a 2.1 system it might be worth it in terms of surround sound.
Will a sound card improve your sound? Maybe, it depends on how good your on board sound is and if you are getting any EMI(electro-magnetic interference). The soundcards have EMI shielding which might help with the analog output, in terms of noise. You probably won't hear any perceived differences in sound but after spending a bunch of money buying a sound card, your brain will probably perceive a difference because it needs to justify the cost.
Once you get over a certain amount of SNR or THD lets say 100, 99.7%(guessing) of the worlds population won't hear a difference.
Even though the Z623 are good PC speakers, they aren't good speakers compared to decent home speakers. People will claim they can hear a difference but if you sat them down and did a bunch of A/B comparison tests then they would probably won't be able to pick out any differences using the equipment you are using.
Personally I feel you are better off spending the money on some better speakers or just getting some good headphones. That will have a greater impact on your sound then a sound card would.