I've 5 boxed with 2gb of ram just fine. I only had problems in capital cities and dalaran but I would just have to move slowly whenever I went there.

"AMD athlon 64 x2 dual core 5000+ 1.8 GHz" would be your bottleneck. I had a 2.6ghz dual core and set my background FPS to 5 and foreground to 20. With that processor you may be able to do 2 fps back and 15 front lol.