If you had asked me 3 months ago if a DAC would make a big difference, I would have said no. I think it definitely depends on the situation/other gear. I have now heard that in at least "some" situations, a DAC can and will make quite a difference. Personally I would not buy an expensive DAC until your other gear is up to par.
For example, I had a Peachtree DAC-it in my office system...PC -> DAC-it -> Emotiva mini amp -> KEF 103.2 speakers. I didn't hear any difference vs the sound out of my PC's soundcard. In this case, I don't think the DAC was good enough or the amp resolving enough to hear the difference.
I also had PC -> Peachtree DAC-itx -> Juicy Blueberry Extreme tube preamp ($2500 preamp) -> Monarchy SM 70 Class A amp -> Quad ESL 57 speakers. Now almost anyone would say that this system should be resolving enough to hear a difference with using the DAC vs not. But again, I didn't hear much difference between the DAC-itX and the PC's sound card output. But I felt this system was "good enough" to justify having a DAC in it, so I had one. I'd like to try this again but I really don't remember hearing much difference, or maybe my ears just weren't trained yet.
Not until this April was I a believer in how much difference a DAC can make. My system is/was the same as in my signature, except it was a Bel Canto DAC 2.5 rather than the 3.3VB. Let's just say I heard a difference! I went from a Peachtree Decco 2 DAC/integrated amp to the Bel Canto, and it was a huge improvement. I liked it so much that I upgraded to the Bel Canto 3.3VB. My system just sounds so "right" to me now. I think I finally had the gear to be able to realize the changes...speakers and amplifier were able to realize the improvements that the DAC provided...in other words, SYNERGY!
In conclusion, it took me about 5 years of upgrading gear and finally spending money on a DAC ($2k new) to realize the difference a DAC can make. Spending big money on a DAC where your other gear isn't in the same league, I don't think you're likely to hear much/any improvement.
Just my experience, YMMV.