If you use editor like Homepage - that I use - you can browse your page in 800x600 without changing anything in your browser (EDIT: I meant in monitor). Very handy.
GG is right. Tables wider than 750 are too wide. You can use some cellpading but not very much. Many times I have first table as 750 with cellpadding=0 and inside smaller tables - if I want to use more than one bgcolor in my site, like today:
http://www.hottyporn.com/cheerleader-pantyhose/
Tables is old way to build today but all those fancier ways don't work with all browsers, so I'm old fashioned and still use tables.
2nd EDIT: (as it seems I can't have my ass up from this chair and go to watch Neste Rally Finland) My recip tables are 600 pixels wide. Every cell is aligned to center. If recip is wider, I give table some more pixels and of course to cell too. Good example is Cleo's Links. Her recip is wider than 200 pixs if you want it to look same as in her site. That's no problem with me, I have 750 pixels and as I normally put 3 recips in a row they use only 650pix max.