My opinion of Compaq/HP is not high, and though I recently bought a dell my opinion of dell isn't too great either. Generally $900 will not buy you ANYTHING that will play Halo 2 (unless they did a wayyyy better job of coding than Halo on PC.)
Your best bang for the buck will be at Dell though. Especially when they have coupons and you watch the sales. If you absolutely have to have a 64bit processor in a laptop you might try Acer. The best you'll probably do without buying refurb is an ATI express 200 integrated graphics. Only a slight bit ahead is some form of a Inspiron 6000 with an X300.
And as far as AMD vs Intel on the laptop front, your best bet is to go for the best bang for the buck and not worry about brand preference. It seems these days that Intel actually has the edge at the moment.