1) Create one site with resolution 800x600 and use relative positioning to place it in the middle. Not pretty but you certain that there is no scrolling, it will happens only if somebody still use lesser resolution then this ( I do not think there is anyone)
2) Use browser detection script that can check on user resolution and use css file assign to this resolution (you need to buy a plugin or you may want to code it)
PS: Linked site is official documentation by w3.org ;)