That is hard to say, because it will vary depending on how firm you like it and how firm your springs are. I would say that you probably will want 3-5" of foam on top, assuming you are taking off all of the foam.
Right now on top of my fairly firm Sealy springs, and I like my mattress a bit soft and cushy for side sleeping, I have from lowest layer to top layer:
2" of 32 ILD 100% natural talalay from rockymountainmattress
1" of 5lb. Sensus memory foam from Overstock
1" of 24 ILD 100% natutal talalalay from rockymountainmattress
1" of 4lb. mem-cool memory foam from Overstock.
I also has a wool topper that put on top as the "cover" and temperature moderator (from Walmart, but there are plenty of others).
This is probably slightly too soft and I am thinking of replacing 1" of the memory foam with 1" of firmer latex (around 19 ILD). Although, I prefer this over using just 4" with 1" less memory foam.
I am not sure if I need the 2" of 32 as a base, but maybe it good to have that. I have thought at times that I would have started with 2" of 24 and maybe 1" of 32.
So, you may want to start with 2 toppers. One a bit firmer like a 32 and around 24 . 1-2" increments are probably best to allow fine tuning. If you like a softer feel, then you probably will want something softer than the 24 above it. The memory foam is pretty cheap, and can add some comfort but not much in the way of support. I have found that the Sensus is better but too hot for me on the top layer.