I second miles01110's answer. Get what you can afford. I have 6Mbps AT&T FastAccess DSL, and it's just fast enough to stream NetFlix SD consistently, and HD during non-peak hours.
MSNBC.com's front page links to an article about NetFlix. It's now the biggest user of bandwidth during peak evening hours, beating regular Web surfing and YouTube easily, and even rivaling P2P/BitTorrent occasionally.