Often what looks great with Internet Explorer falls on its face with Netscape. Telling your visitors what browser is best is nice, however they are not going to open up another browser just to view your page. Simply reduce the size of your open window to see how it will look at 640 x 480. Most people do not like having to scroll horizontally in addition to vertically. Also, be careful about color schemes.
Colors are great for making a plain site look great.
Another point is the use of frames. Please don't employ frames without giving your visitors another option. For some older browsers frames simply slow the site down too much for people to hang around. Really and truly, people will avoid sites with frames. I understand that some sites make navigation easier with frames, but if it's not necessary, the added effect will cost you.