Personally I see no need for external converters if your using the 2408mkIII. The converters that're within the unit are excellent unless you have intentions on doing "high res recordings". In that case you would have to look towards a 192k TDM setup or the motu HD192 interface. But you should be just fine w/ the 2408.
Thanks Opus for bringing me up to par. I did'nt know the MOTU converters were that bad. I've always thought & heard that their audio interfaces were the shi_. I know the apogee products are very good because I still have one of their models on my wish list. Good look
Here's a quote from behringer: "Removes jitter and corrects incorrect sample rates". This is from an advertising clip I ran across about their SRC2496 Ultramatch Pro AD/DA converter. Don't know how well this unit is , but I'll do a little research on it.