I would go for 2x 4870s purely for the flexibility and even portability if you build a new system. The 4870x2s are literally two cards stacked together in crossfire.

A 4870x2 would probably be slightly more power efficient compared to two individual cards, but not noticeably. Having crossfire enabled could definitely impact performance some for applications not designed to take advantage of it - it would only be extra stuff in the background.

Cooling would really depend on your setup and case. Running 1x 4870 while playing would be cooler than the 4870x2.

That score does seem a little low but it would be very safe to blame the artifacting.