Yes, that's exactly the decision If yo don't know what to do, I suggest using 40-24 if you're gonna install many programms, or 48-18 if you don't really need many progs. In both cases, you could probably optimize your usage, but to find that out you'd have to use it a bit to know what you want...
Uhh, yeah, to clarify: the memory of 64 MB is split for RAM and storage. Storage can mainly be used for programms, but other stuff as well. You probably know what RAM is... (I guess that's what you meant in the first place, just wanted to make sure).... Hope that helps!
Cheers
Philipp