I have a Beresford DAC with my RA-05 which sound great together. Depends on your budget I suppose, but I went with Beresford as reading around showed them to be among the best under £200, and many people compare the quality to much more expensive DAC's.
I have the Arcam rDac with my 1062 and like the punchy sound that it has. As JonnyD says depends on how much you want to spend. But it is also worth considering which input you will use most, if its usb from a PC or Mac then a DAC with an Asynchronous usb input (which the Rega doesn't have) may sound better, such the rDac, dacmagic plus or even the HRT musicstreamer. The latter is also usb powered so no need to connect it to the mains.
Depends what you're going to do with it. If it's for streaming, fair enough, but if it's to improve your cD player I'm not sure I'd bother - 500 quid on a second hand amp would have a more profound effect.