modest DAC for iMAC+iTunes?

I have a new iMAC which I currently connect to a vintage Marantz 2250B for my bedroom system. We use it for iTunes, playing CDs, and streaming internet radio. I use analog cables with RCA jacks.

I would like to try a DAC for the best sonic quality. I think I can get a lower noise floor. Can I get something for < $200 that would work well or are we talking more $$?

Not sure what the jitter performance is of the MAC digital output.

Any recommendations?

Thanks,
Rob