I don't know much about the technical stuff but is running out of memory the most likely reason for this results? Or is 4x0 simply faster? ~200MB diff doesn't look much, but like I said, I know little about that.
Not to burst your bubble here ;-), but shot of HAWX above is still on average faster on 5870 than 470(i hope thats 470 and not 480...),max fps are higher too, MUCH lower min fps however suggests that 5870 runs out of memory in few places ;-)