There are many examples on the net of how to do rounded corners using css. Here's 3:
http://www.alistapart.com/articles/customcorners/
http://kalsey.com/2003/07/rounded_corners_in_css/
http://www.albin.net/CSS/roundedCorners/
Perhaps you are using a table because you're not accustomed to css, but it's definitely worth getting into css as it can make your pages smaller and load faster and you can do much more with css than with tables and spacer gifs.
try the following to see what css can do for you:
http://www.s7u.co.uk/
http://www.wow-factor.com/ (has links to many great css sites)