If you are already in AVR, did you look at the XMEGAs ?
Only a small jump from Mega48 in SW terms, and they have 12b DACS and plenty of RAM.

Or, if you _really_ want high performance Sine saves, try something like this

http://www.cirrus.com/en/produc...ey=CS47024

That has 108dB precision Audio DACS, and a DSP all in one package.

One way to get higher precision, (smaller steps) in a Fixed Frequency generate, is to vary the samples per cycle.
(that does need new table calculates for each frequency)

If you assume a 1KHz sine from a 1000 sample scan, that same 1MHz rate can generate 999Hz with a 1001-baseline scan, and 1001 Hz with 999-baseline scan
Give the scan more memory, for 10 whole cycles in 10k samples, and now you can resolve to 0.1Hz, with no jitter.