The 8GB of ram, should be plenty for 5-boxing.
Not sure how fast the ram is, DDR3 was a huge gain over DDR2 when I upgraded the processor, motherboard and ram a while back.

The 8800 is not a particularly strong video card, but most of warcrafts challenging features strain the processor more than the video card. Shadows, water effects, weather effects and spell effects push the video card. Almost everything else is more of a drain on the processor.

A dual core, not sure how well that would run five accounts. I was using a Q6600 up until two years back, and that would run five toons fine in most areas, but I was not happy with performance when flying on epic mounts, in alterac valley or in the cities.