The ACA1220 will undoubtly make your stock A1200 faster:
Standard A1200 = 1.37 SysInfo Mips (aprox)
A1200 + ACA1220 = 3.48 SysInfo Mips (aprox)
StandardA1200 = 2MB of chip ram
A1200 + ACA1220 = 2MB of chip ram + 128MB of fastmem
Not only the mips (cpu) is important regarding speed, because with that much fastmem on the ACA1220 you can probably put all your system structures on fast memory (your entire System might also fit there), and you will get this way an added bonus in improved performance.
ACA1232 vs ACA1220 issue: Besides speed and MMU (this last one useful on some rare ocassions), I guess there isn´t something else that makes the ACA1232 stand above the ACA1220.
