If you can swing the coin, I'd go with one of Steve's routes over the simple converters. A good friend whose ears I respect heard a demo of the PaceCar at a show and was very impressed. If you have to go with a less expensive option for now, most definitely do not use the Airport express if you want decent sound for sitting down and listening. I've tried to make the AE sound good with several different DACs, but alas, you are limited by the unit itself and the conversion circuitry within the unit that gives you the S/PDIF (toslink mini) assuming you're going for the best sound from the stock unit. I have never been able to get sound I wanted to sit down and listen to from the AE (stock) - if it's background music you are after, it's as good a solution as any and you probably don't have to spend any more. In any case, if you are looking for the best sound from a $200 investment in what you already have go with a decent converter and your Benchmark. Just my opinion.
Marco
Marco