I missed the point, you won't use a 8051, just the eprom and counter.

Your approach is ok. Why not 4 waveforms (add tringle)? You can have a little logic circuit so select the aproppriated memory block using the two higher address lines (00, 01, 10, 11, you've got it...) so your counter can have just 1/4 of the size of your eprom (smaller counter= higher frequencies).

By the way, the buffer is unecessary, the dataoutput of an eprom can drive the DAC inputs. So your design can be even simpler.

hth

orlando