It's too early to talk about NeoGeo requirements. AFAIK author is in evaluation stage of requirements.
ao486 doesn't use SDRAM module. It uses DDR3.
Atari should be happy with 32MB as it was plenty on that time. 8MB on that time was like an unlimited amount
As for chips, SDRAM in this chip format was 64MB max. Using 2 chips is probably impossible due to I/O line load and signal propagation time. The fact that even current SDRAM module works is just lucky coincidence and my perseverance. It was hard time for me to make it work on GPIO connector originally not supposed to drive such fast device. I've made several prototypes in order to make it work reliably on >64MHz clocks. And so far i found only one SDRAM chip working on acceptable speed - Alliace AS4C. Much later i've discovered another chip Winbond with similar characteristics. All other chips are failing in these conditions.
64MB chips have worse characteristics in terms of power consumption and thus have lower max clock according to my tests. I could make it work only at ~130MHz which is ok, but very close to clock used in some cores now.
Minimig is the only core using full page burst mode (X68000 too but this core is almost useless), so in order to work with 64MB chip it has to be modified as 32MB uses different row/col addressing. All other cores doesn't care of addressing mode and can work with both 32MB and 64MB chips.