You need 768MB of RAM for Android 4, and realistically a decent GPU. 256MB and the most basic $0.50 ARM chipset from Qualcomm [1][2] will get you up and running on Android 2.x.
Yes, it is a bit, but not like iOS6 would run much better on these devices anyway. If the new chips can use some decent GPU's the experience should be significantly better on ICS/JB than on Froyo, though, because the UI wouldn't use so much CPU time anymore, and wouldn't choke as much doing regular operations.
