well do you have any source component for digital, or does it need to bring that as well?
also, just to throw it out there, if you're just gonna go with a DAC, you'll need an amplifier, as line level into Grado's probably wouldn't sound so great
I'd look at a combo unit in the $150+ range, or go even more complex and get a full on audio processor, like the E-MU 0404 or one of the M-Audio FastTrack units
the benefits of an amplifier with Grado hp's, imho, are just increased control, it doesn't really improve "can they get loud", it just means more controlled sound, the D/A will improve clarity (as dallan has said) and resolution, having both in good shape will do wonders