Sounds like the famous Nvidia 8600M GT failure right there. I had a 2007 SR MBP that I sold in 2013 because I was afraid of that happening. It affects both the Mid 2007 MBP and Early 2008 MBP since they use the same defective GPU.
It's on the logic board so it isn't easily repairable. There are companies that will reflow the solder for you however since that's where the problem lies (it cracks from heat/cool/heat/cool). I did manage to fix a few temporarily by using a heat gun and reflowing it by melting the solder and getting rid of the cracks, but they will inevitably come back some number of months down the line.
I did do a lot of preventative maintenence on the one I had though, including replacing the thermal paste with some AS5, and lubricating the maglev fans and using fan control. Anything to keep it cool. But it's pretty much past that point now.
Any money would be better invested towards a newer Mac, even a used 2013 model or so.