The $120 4870 x2 does run hot, but it is half the cost of the 6950. You would have to compare it with a 5830, 6850, geforce 460. I have a 4870, not the x2, and they are pretty fast cards. If you have the p/s to run it, it would be ok. Most early 4870's and x2's had to run underclocked to make them stable, so your looking at 700 core/850 mem, default is 750/900. It ends up being between a 4850 x2 and 4870 x2. Most newer games support crossfire so it should be fine.
At stock speeds the 4870 x2 is faster than the 5870 in games that support crossfire. The 5830 to 6970 cards are crossfire gpu's made on one die. amd just put 2 dies together. so games that do not support crossfire also only use half the shaders on the 5830-6970 cards as well. This is where the nvidia geforce has a advantage over amd. But it comes at a price of heat, power consumption and noise. (both the x2 and fermi).
If your power costs over 10 cents/kw/hr then get the newer power sipping card,
If your power is cheaper where you live than this card could save you some cash.