Yeh, this problem's been around since the dawn of mankind. It's there because website designers always assume American users and that therefore visitors to those websites will always be using American letter-sized paper for printouts. That's, of course, not the case, as we in Europe use A4 paper size, which is narrower. Thus, unless the particular website designer has been sufficiently thoughtful about this, we Europeans are often stuck with portrait printouts with the RHS cut off.
I'm afraid that, most times, altering the margins in Page Setup isn't a solution. Very few printers give you Fit to Page, either. The only way you can really guarantee not losing the RHS is to change to landscape mode for printing (someone's already mentioned that) but doing that seems to waste more paper than is necessary and also, quite often, text will get truncated across the page boundaries. Also, I've found that, once you've set the printer to landscape mode, you have to completely exit the browser and then open the Printers applet again, to reset the printer back to portrait mode. Resetting the printer while the browser is still open doesn't put the mode back to portrait permanently. Aaaaargh!