Hardcore Ajax fans are in the lower tier behind the goal at the opposite end to the away fans. This is the only section of the ground where you'd be ill advised to show your support for United, anywhere else you should be OK.
For big games, the best atmosphere is in the bars around Rembrantplein in the centre and also in the red light area, the Old Sailor is a good venue, also the imaginatively named Red Light Bar, you'll get a few Ajax drinking around here too. By the stadium you have a few bars in the shopping area between Bijlmer train station and the stadium, next to Heineken Music Hall, which are always busy on match days. The Ajax Supporters Home is also very busy, near the training pitch on the way to the ground from the metro stop.