Smooth transparency with gif's? (Photoshop)

[MagSafe]

Hi there,

I'm trying to create a white curved footer image and save it as a transparent GIF, so you can see the background instead of a static colour ... but when i'm using Photoshop and 'Save for Web' the curve looks really jaggy when loaded onto the webpage.

Is there some settings I should be applying to create a really smooth curve with lots of anti-aliasing using Photoshop?

Thanks