At current rates, 5 million U.S. is worth 3,848,520 EUR
So Uncle sam (every working tax payer in america chips in to send you packing) gives you 5 mil
Given that it's very unlikely you'd be allowed to work in the EU, and your net is 3.8 eur and change, is that enough to keep you alive without working?
The first thing you get hit with when you buy your house, is IVA/VAT to to the tune of 16 percent on the house. but thats chump change compared to whats coming.
Royal Decree 687/2005 is specified as a special expat tax in spain. While it may be lower than the original 43 percent, there are a lot of hidden elements to this.
You'll pay 30 percent upon declaration of intent to establish citizenship. You'll also be subject to NRIT the first 183 days.

I estimate you'll go through .5 million euro in those first 183 days by the time you get a house car and all filings settled.
Your then on your happy socialist way to being a spainard with 3.3 million Euro left. So lets say your going to live another 30 years.
Thats 33K Euro a year. It cost a lot of cash to live in Europe, especially spain. That 33K will be eaten up in a variety of taxes and fees not to mention just the cost of living which is ever rising. Then there is that whole socialist thing they are trending to more so than the U.S.

I figure you could live good for about 10 years before your out hawking that CLK 500 wishing you'd listened to people that had common sense.
Even with the trend towards socialism in the states, your still better off here than under the extreme taxes common to Europe.