I have a few questions before we get to the nuts and bolts here.
Why did you throw out a perfectly good 8600 just because it was overheating?
Why would anyone run 2 -8600's in SLI in the first place?
Why would you buy a GTS250 and try to SLI it with a 8600?
I ask these questions because...
An overheating card is easy to fix, it is almost always not a problem with the card at all, but more a problem with your setup, the card is full of dust, or the fan on the card may have quit working, you are pumping too much voltage to the slot, or the airflow in you case is simply bad. Anyway, all easy to fix.
A single higher end card like the GTS 250 will outperform 2 weak cards in SLI, like the 8600's
And lastly, to SLI 2 cards, they have to be the same card, or family of cards at least.
I did not think (and I could be wrong) you could SLI a GTS250 with a 8600?