Illustrator "Save for Web & Devices" returning crappy, pixelated images

Question

I'm trying to create a nice title for my webpage... a big white title to sit on a black background. I'm using Illustrator to do so. When I create it, it looks nice, but when I hit "save for web & devices", it comes out looking like a pixelated piece of crap on the site. Is there some setting I need to change to make Illustrator save a higher resolution image? Thanks

EDIT I understand, from looking at some other posts, that this may be a result of "posterization" or "dither", but this is only a plain white image so I don't how this results in a colors problem. (I could be completely misinterpreting these terms)

EDIT Figured version might be important... I'm using CS5.1

Solution

Below the preset area, on the save for web dialog, there should be a small tab that says "Image Size" and in there, a small panel says New Size and percent, try 200 percent or higher. don't forget to hit the apply button.

You may also try the 2 up on the upper left corner so you can compare them side by side.