To have rounded corners in ie6/7 create another css file ( and create a img with photoshop with rounded corners is simple ). If the client visit your site with ie7, will see rouded corners becouse that box will have a background-img ( that img that you have created ) i hope you understand.
I best way to go about this is use rounded corners in your CSS. Then for IE, just use a separate CSS rule that uses images. This will help you save bandwidth. If 30% of visitors use IE, then that means the images used for rounded corners will only be requested 30% (saving you bandwidth) . Like mentioned in the previous posts, it's like "dumbing down" to IE.
i use css3.0 methods and for ie a javascript wich make's css3.0 possible. is that good for bandwidth?