PG is recommending you get the X.com, but that doesn't mean you need to pay $50,000 for X.com on Day 1 of your startup. I see people making this mistake all the time, buying expensive domains before validating their startup in the first place.
Think about what Dropbox, Stripe, and AirBnB did!
getdropbox.com -> dropbox.com
devpayments.com -> stripe.com
airbedandbreakfast.com -> airbnb.com