I had watched a whole bunch of stuff on the G4 network about the Sims and the creator of the franchise (along with SimCity, one of my favorite games). So yes, that's the #1 Selling game of all time, now, "officially". Still can't find a link but it's true.
And I dunno about UT 2003 being so popular. Sure it's a great game but the system requirements to really run it smoothly are relatively steep. Not to say it won't be extremely popular, though. Just maybe not so much as Half-life.