Well, in that case, assuming power consumption is proportional to the square of clock speed (Which I believe it is) then:
To get a 3870 to ~60 Watts we are looking at underclocking it by ~30%.
To get a 3850 to ~60 Watts we are looking at underclocking it by ~20%.
20% seems reasonable.
That means, just going on core clock speeds,
3870; 780 MHz UC -> 550 MHz
3850; 670 MHz UC -> 540 MHz
And then of course, the HD2600XT has itself been underclocked by Apple. I don't know by how much, but at this rate, it is still looking to me that we won't be seeing one of these cards. After all, these cards are full-on gaming cards, and the iMac is not a full on gaming computer. I think a more realistic expectation would be hopefully something with 160 stream processors (Half one of these cards) and GDDR4 Memory. Maybe we'll get one with 240 stream processors, and underclocked by ~10%, which would be a good compromise. But I would be betting the next iMac will come with the HD3670 or HD3770, at AMD recommended clock speeds, which should be enough to run Crysis at a decent res, everything on High. It would of course totally own the HD2600XT, even the X1900XT in today's Mac Pros. Hopefully not for long, though. Apple needs Crossfire HD3870's.