External DAC (and especially when combined with an external headphone amp) benefit the listener, when driving harder to drive (insensitive), power-hungry and impedance-variant headphones. Driving such headphones from, say motherboards headphone output, may result in less than stellar audible results.
So, That's why I asked about the headphones, because which headphones you use does matter. With certain type of headohones you can actually benefit from an external dac/amp (* there are other potential benefits from an external dac-amp, like measurably better S/N, better external noise suppression, etc).
HE-4XX is a fairly low sensitivity headphone (at 93 dB/mW) at a fairly low impedance of 38Ohm. Your current X-Fi Titanium HD has an output impedance of roughly 32 of Ohm (way too high for a 38Ohm headphone like HE-4xx) and output power in few milliwatts even for easy loads. This will more than likely result in changes in frequency response, as the headphone is not a linear impedance load. In plain english, worse sound quality than what the headphone is capable of with a proper amp.
Thus, your sound, esp. bass performance will benefit from changing from Titanium HD to a low impedance (near 0 Ohm) output and more powerful (a hundred milliwatts minimum) DAC/Amp, that is able to drive even lower impedance headphones (like HE-4XX) that are harder to drive due to their insensitivity (again, like HE-4XX).
You don't necessarily need an external DAC though, just a better headphone amp connected to your soundcard, is enough. The amp will then give low impedance output and more power to the headphone, better than the soundcard itself can give.
In plain English, your sound quality, even with current headphones, will benefit from an external amplifier like for example,
Topping L30 (c. $140, used with your current soundcard feeding it analog signal) or if you want combined DAC+headphone amp, somethin like
Schiit Hel (c. $189). These are just two examples. Budgets and preferences from people to people vary.