Not sure, perhaps make it out of more "industrial" materials and forget about temperature? Who cares if it gets to 150C if it's only going to be purchased and operated by "professional miners" ??
You can't run 5870 chip at 150C for a long time, no matter what materials are used for other parts of the board.
And 5870 chips are not being produced now, AFAIK.
And you have to get this chips from ATI - it could be a problem for a small company.
And energy efficiency of this solution is much worse than FPGA.