Which would be a better option to go with a creative sound blaster audigy fx 5.1 soundcard,or a Creative sound blaster x-fi hd amp/dac?
Are you using headphones or speakers?
I would say keep your onboard sound, you won't gain much from either of those 2 options unless your integrated audio is broken.
Both I have a 5.1 speaker setup,and 150ohm headphones
Unless your on-board sound is really messed up (buzzing or clicking that only happens with the PC, too quiet or the software does not do what you need it to) There is no reason to go with either unless you are using high-end headphones or speakers. In which case an AMP/DAC is going to give you more isolation from "noise" from the PC. While a sound card is going to have virtual 3D sound stuff that may or may not work.
Either one is only worth anything if it is pushing some decent cans or speakers. So ask yourself how much you want to spend on that before you look at cards and DACs. And good surround sound systems will have a head unit that accepts fiber optic digital input. So it is doing the AMP/DAC work itself.
Try using your rear plugs if you are having audio problems before buying anything. Almost always it's the front panel plugs/wiring that are the problem.
That said, buying a ROG board with good on-board sound with a preamp has really paid off for me on my Sharkoons. And my brothers PC360 headset from Sennheiser is stunning. (he has same board).
It's kinda like blu-ray HDMI and HDTV. There is no reason to own one without the others. If you are watching DVDs on a HDTV via HDMI or blu-ray on a CRT via RCA you are not getting what you paid for. You need the whole high-end package to get the experience and for some people it makes no difference at all.