I know where you are coming from OP.
I just recently "upgraded" to the i5-3570k +UD5H ($346 total from Microcenter: MC) from the i7-920+UD5 due to the very reasons you have; heat and power consumption. At the same time I also took in account the current value of the i7 920 (paid $213 at MC) and was able to sell it for $175 on ebay about a month ago (it's all about the presentation). Also, I wanted to take advantage of the full performance of my SSD (Kingston Hyperx back in Feb), even though i noticed a improvement on my x58 from HDDs RAID0, i still wanted the full performance. And the MC deals on the SB and IB was another thing.
So all these factors were my valid reasons to justify my "upgrade". Even though one might say you are losing money, its all about getting your money worth from the years of using it and being able to recoup some money to fund your next setup. for me, in the long run the "upgrade" I think its going to be worth it. So far its has especailly being able to use my Vertex 4 SSD to its fullest.
I remember the heat being produced from my i7-920, and during the summer it was really hot, felt like my legs were on fire since my system is right next to me, with the rear top fan venting the heat out. lol
Since you have access to microcenter, then you can take advantage their CPU+mobo deals (AMD or Intel) in which you are getting a pretty good deal.
For the HD5870s, i would appraise them for $145-$165 if you plan to sell them. Edited by Raiden911 - 6/26/12 at 2:28pm