when to use what lens.. ?

I've been using my dads Nikon D70 and I got a polarized lens for it and a hood, but my question is when is a good time to use either or, and should I ever be using both at the same time? I've been shooting a friend BMXing lately and we shot all day yesterdya and it was sunny in the morning so I thought I'd experiment and use both the polarized lens and the hood, but when I put the pics on the comp they look... I dunno not as crisp and colorful as I was hoping for. They almost seem a little grainy but I think that may be cause of the polarized lens? Later in the day the clouds rolled in and I took both off, the pictures seemed cleaner but with the grey sky the pics still weren't what I was going for. So when should I use the hood and when should I use the polarized lens? Also I've been saving for a Fish eye lens.. is it worth the money and does anyone know a cheap place I can get one? Are there any lenses that you can use on a cloudy day to make the image.. i dunno... more colorful?