Some slots can't have GF, like the weapon and offhand. There are some legendary weapons with magic find on them, like the blade of prophecy, normally a weapon also can't have magic find.
So you'd get 10x20 + 40 + 25 + helm gem for MF, 10x25 + 40 + helm gem for GF. The top gems are so expensive you'd probably use a lesser one, so around 250-300 becomes a practical maximum.
If you're talking about effectiveness, developers have hinted that MF has diminishing returns similar to diablo 2, where the effect of MF increased more slowly above 100%. Gold find seems to be a straight multiplication of drops, 100% GF is twice as much, 200% GF is three times as much.
ive put a nice collection together, and with all my gear on I have 305%MF. (not including using enchantress with items, or having NV stacks.)
but to get that set cost me a LOT of AH hunting for good bargains.
the only item I can upgrade now is my rings, which are at 17%, but i think the max is 18% on them and the gem in my helm.