2560x1600 pixels is 4.1MP. At 1:1 that will completely fill the most monstrous computer monitors one can get for under about $10k. At 300 pixels per linear inch (a very reasonable resolution for photos; about the same as the iPhone 4 "retina" display) it will give you a picture with 10" diagonal.
Unless you're making posters or big prints for photographic competitions or something, even at 4MP raw pixel count is not going to be your problem. It's completely untrue that below 6MP your pictures will be useless for anything beyond Facebook.
(Of course pixel count starts to matter more if, e.g., you're taking pictures of distant birds or distant celebrities with a not-especially-long lens and you need to crop heavily. Most photographers, most of the time, are not doing that.)