If you can afford it get the dCS Purcell 24/192 upsampling unit. If you are on a budget pick up the Perpetual Technologies P-1A. My buddy has the P-1A and P-3 DAC combo and it is quite refined.
Somewhat depends on your price point, but I guess if you're entertaining the dCS suggested above you've made price a moot point.A single box unit that I currently own is the Audio Aero Capitole, which does 24/192. They also have a DAC unit which does the same. This is an incredible sounding piece of gear. I previously had a dCS Delius/Purcell combo, which was quite good. I prefer the Capitole, I haven't heard digital get any better than with this unit. It also has about 3.5 V of output and a volume control, so you can drive your amp directly, bypassing your pre-amp, if so desired. The results of this must be heard to be believed.