A memory buffer in the DAC can help with jitter depending on the design. One that is properly implemented is in the new Benchmark DAC. The downside is that you must live with the sound of the Master Clock in the DAC, which may not be to your taste. Also, if you give the DAC a really low jitter signal, the result will still have the intrinsic jitter of the clock inside, which may be more than the incoming jitter.
Many inexpensive DACs now come with reclockers inside. The problem again is that these are usually not the highest quality clocks or the best implementations so the result can be hit and miss. Even with these designs, the input jitter still matters. You can tell just by changing the coax cable or the source. If it sounds different, then the DAC is not immune to incoming jitter.
99% of DACs will benefit from a good reclocker like the Synchro-Mesh. Those that do NOT have reclockers already inside will benefit the most.
Steve N.
Empirical Audio