Hmm 30-60% while good isn't quite what i'm looking for. I'll probably wait and take another look at things around winter time. Hopefully by then we'll have a refresh/die shrink from the green team, and some nice price cuts on the 4870X2.
P5K is a single slot pci-e2.0 x16 right?
i'd say wait for 4870x2 - but make sure you have a good PSU for it; i'd expect 650watt with 36-40amps on the 12v is good for the card and your overclocked processor if you already got it. Might even leave a little headroom to overclock the GPU.
I think it also requires 2x 6pin pci-e? but it might be 1 8pin 1 6pin, not sure on that