You will want to create an image for the rounded corner. I have done quite a few of these and although I prefer Adobe Photoshop, pretty much any graphics program should suffice as long as you can verify that you're using a web color for the graphic.
Make the curve you want uses the background color you will use in your table. I usually start a page template by simply drawing the complete layout the way I want it in Photoshop and then simply cutting out the portions (such as corners) that need to be graphics. This way you're pretty sure that everything will fit together right and it's easy to figure out your cell dimensions.
Size really depends on your layout. You obviously want a quick load time so the smaller you can make it the better. After looking at golf.com, I'd bet that the top border with the curve is it's own table and they start another table immediately after for the rest of the page. You probably know that tables will bump right up against one another and look continuous. I do this quite often. This way you don't have to worry about putting the corner image in a cell that works with the rest of the page. The corner on golf.com is 19x18 pixels which means that it is just big enough to get the curve and the rest is taken care of by background color.
Hope this helps. This is something that I just got down over time and it still gets easier everytime I do it. [sig][/sig]
