You don't get everything. In fact you get almost nothing. You can rewrite recent blocks which just means that you can unpay coins you already had control of. Or you could refuse to put transactions in blocks or to build off of anyone who does effectively a DOS. But you don't get $20M by any means.
How about if someone with a lot of bitcoins rented a machine like that out for a short period?
You could then cash in on the bitcoins several times over?
What would/could happen to bitcoins after such an attack?
You need to distinguish honest mining from hostile mining. Honest mining is when you always build on the longest block chain. Hostile mining is when you reject blocks for nefarious reasons.
If someone rents some hardware and mines honestly, that's great, he strengthened the network and is rewarded for it. Note that when the mining reward will consist only of transaction fees, speeding up block generation will cause each one to be worth less.
If someone used the computing power to reject blocks, there's little he could do to profit from it. He can damage the network, but not catastrophically.
