I'm just one click away of buying me the new iMac (20') , but there's one thing left of which I'm not sure: whether I should get the hard drive with 250GB or with 500GB. I want to use it as my main Windows XP machine as well. So should I get the 500GB one, because there are gonna be two operating systems installed or are 250GB enough? I do have an external 250GB hard drive. Any advices?