I will just state that smcFanControl is not a smart thing to have installed. ESPECIALLY if you use it to try and override fan settings. It's one thing if you use it only as a monitoring tool, it's a whole other if you try and change settings. Apple will never officially say it, but I can tell you that they also unofficially frown upon its use. It's something of the bane of techs everywhere, because it can cause false-positives during diagnostics, which just wastes our time and makes the repair process that much longer.
But if it's an early 08 and you've got a known bad nVidia graphics chip, exhibiting problems, odds are real good you'll be able to get it replaced under the extended repair program. Just don't let any place try and charge you for it. Apple pays labor on this repair to the AASP, so they aren't supposed to charge you for it. If they try, you can tell them to check Apple's extended repair program list, and this will be covered. If they still try and bill you for something, take your system somewhere else and call Apple to report the place you were just at. Odds are they've been screwing over other customers, and Apple would definitely want to be aware of that to potentially revoke their AASP status.
Also, you probably don't want to wait overly long to get this looked at. I've done a couple dozen logic board replacements on these, and other afflicted models. Plenty of times, the chip just flat out gives out and you lose all display capabilities. The diagnostic program is designed to handle this eventuality but, it's annoying for you.
Finally, I do see the Early 08 with 8600GT as being one of the models indicated by this program. So if the repair place isn't aware, tell them to look up KB article TS2377 which will outline everything they need to know. Including how to get labor reimbursement from Apple. So again, if after that, they still try and charge you for anything, don't pay. Go somewhere else and report the place to Apple.