Gonna correct a few things. The HD 5970 is 2 HD 5850 chips (x2) and usually overclocked. Some say if you have a 4870x2 to not go for a HD 5870 +. The 4870x2 has more power, yes, but the 5870+ are similar in raw power and faster in general cards with more streaming processes and require less wattage to run.
The 5870 on general seems to cost around $400 almost everywhere. The 5970 seems to cost $600 mostly. On top of that, a lot of games have issues detecting dual gpu cards such as the 4870x2 and 5970. Personally I would go with a 5870 and crossfire it.