The jaggedness of the rounded corners has to do with the 1-bit alpha GIF files work with, it doesn't allow for blended transparencies. A pixel is either fully transparent or fully opaque. What this means is that what looks like a smooth curved line in photoshop gets that jagged edge when it's saved as a GIF because it has to cut the curve into perfect square pixels. PNG-24 is my prefered choice for static images with transparency because it can do blended transparencies.
Quality wise, for Sigs and Avatars I use the highest quality possible within the constraints of the system. Honestly, you could make the most complex 550x150 image possible and without animating it's not even gonna be a bump on the road for the standard broadband connection.
