I have been using Creative cards for years in many builds, and have never had any IRQ, driver, or PCI bandwidth issues. My experience with the drivers and the X-Fi has been mixed, as for some reason I run into issues whenever I use the drivers on the Creative website, but all is well when I use the drivers straight off the CD. However, you can choose to install the bare drivers (no bloatware) only, which gives you control over all the basic functions you should need. This is all on XP, though- no experience with Vista.
As far as quality vs. onboard, there are those who say the latest HD onboard is comparable to low to mid-end cards. All of the Realtek onboard sound I have heard to date (most recently on the Gigabyte GA-MA69GMSH2) has been quite terrible compared to the Audigy 2 ZS, much less the X-Fi. That is not to say that the Audigy or X-Fi are great, but they are, in my experience, much better than onboard.
One benefit of the X-Fi is that you have the option to run bit-perfect over coax from the flexijack. So, should the day come that you are no longer satisfied with the sound quality of the X-Fi and want something better, you have the means to pipe bit-perfect audio to an external DAC.
You would be hard-pressed to do better if you are looking for an all-around card for <$70. In the $100 range, though, there are arguably better choices.