If you wish to post the code or pm me I could try to work somthing out but what I am thinking at the mooment (i know, I'm being dangerous again

), is that you could set a div to fit 100% of the screen at 1024. Within that a centered container div of xx%, which would equate to 932px. Then as you reduce the resolution to 800 x 600, the 932px equivalent would reduce proportionately, to fit in the 800x600 screen.
What I can't tell yet is, how would it look in the 800 viewport at that size. And I haven't calculated what % 932px is of the 1024 resolution.
Have you considered how much easier it might be to redesign their site?
Might be worth considering because if you design the homepage in symantic code, then it might just be a case of copying and pasting to apply that new code to all their pages. just a thought. Also, it would give a better value finished product for the client too. Having said that, my cynicism suggests that if they only want you to design the homepage, maybe they are going to apply the code to their other pages themselves?
Bazz