What good would giving 250 million people each $640 do?

You say that, but something a lot like that already happened: https://en.wikipedia.org/wiki/Economic_Stimulus_Act_of_2008

Most taxpayers below the income limit received a rebate of at least $300 per person ($600 for married couples filing jointly). Eligible taxpayers received, along with their individual payment, $300 per dependent child under the age of 17. The payment was equal to the payer's net income tax liability, but could not exceed $600 (for a single person) or $1200 (married couple filing jointly)

Most everyone I knew spent it on HDTVs. Economic stimulus indeed.