I just converted my etrade acct to margin a few weeks ago, and it is a $2k minimum to convert.
Once you've converted (or opened) the account, if you fall below that threshold you can no longer trade on margin, but as Gary said, you still get the instant settlement of funds.
It's worth the switch imo .... having instant clearing of funds is insanely beneficial, especially if you don't have a lot of cash to throw around into various plays. I can't tell you how many runners I missed waiting for cash to clear, and how I've been catching pieces of those runners now and making really nice profits
Make sure you don't violate the weekly daytrade limit, though ... if you haven't already, read up about being labeled a "pattern day trader"