Speaking of memory, I've been pondering just how much memory should I put in a recently acquired Macintosh IIsi (it currently has 3MB, which is really unacceptable). 

My basic goal is to run System 7.5.5 or Mac OS 7.6.1 at an acceptable speed, and maybe play some games on it (nothing really serious). I've been thinking of running A/UX on this baby, but I guess I'd need to find a coprocessor PDS board before (any suggestion?). 
I googled a bit and I was oriented towards putting 16MB on it, would it be enough? Should I look directly for a more substantial 64MB upgrade?