The GPS problem was not uncommon with real N900s and was fixed by changing the location info (located in settings) to Google from Nokia. I don't have a N900 in front of me now so I'm not certain of the exact setting, but it was something like replacing www.nokia.supl with www.google.supl.
You are a right with the have to changing. But your address is a little bit wrong. supl.google.com is right.