How much usable RAM does the Wii have?

I'm so confused, but how much usable memory can a homebrew application actually use? I want to load a rather large cache file into memory, but I'm scared that I might be trying to allocate too much.

Well reading this, I can see that MEM1 has 24MB and MEM2 has 64MB. I'm not sure about how much is actually usable.

Aim for about 64MB total usage (it's a bit more but this is a nice round number) and no more than 48MB in a single allocation.