To answer your question specifically, no you don't need a DAC as you can just use the analog out on your soundcard and let the computer do the digital/analog conversion. You've got aÿdecentÿadd-on soundcard which means the quality won't be awful, so you simply need to buy an analog interconnect.ÿ
An external DAC will sound better however. I couldn't find the model you mentioned on the Creative website, they have two that sound similar under the music header (X-Fi Xtreme Audio & PCI Express X-Fi Xtreme Audio) and both cards offer a digital out connection which is what you'd need to use to hook it up to an external DAC. The PCI Express model has an optical/Toslink connection and the other model seems to have a SPDIF/Coaxial. Which you choose to use comes down to cost, how close the pc and amp are and if you're after the very best sound quality you can get but they're all personal decisions. No you don't need a DAC though; however, I'd use one personally.ÿ