There a lot of tools out there for determining filter order-just search the net. I'm not really familiar with the Atmel processors, but I'm not sure this ATMEGA is up to the task; first, it's only an 8-bit processor-that will impact your audio quality. Filters require a lot of multiplication and that device only has one multiplier, so that's going to slow you down.
As a first-order evaluation, I'd calculate the required filter size, and then estimate out how long it will take to calculate each sample output. (It's just a bunch of multiply-and-adds). Maybe if 8-bit resolution is acceptable, and you don't require too much bandwidth and the processor doesn't have to do anything else but filter, it might work.