The nvidia global driver settings that have caused problems and given me CTD's on patrol startup include some fancy advanced anti-aliasing settings and one or two other ones. Just do a search of the forums and you should find plenty of tips about the right settings.
As for RAM. My secondary/work computer has 1GB RAM, 2.4 ghz single core hyper-threading proc, nvidia 6800 AGP video card and it used to run SH3 just fine, or even just as good as my new one. Never had any problems. There is a freeware program called Memturbo that you can use to free up memory before firing up SH3 or just to see if there is not enough for SH3.
Your 1.8 processor -should- be able to run the game imo. If I were you I would troubleshoot the problem first and if it turns out that the processor is too slow, I would overclock it
However, I think 65-75 degrees celcius is still a somewhat high temperature for your video card. It should not go above 60-65 imho. An easy and cheap way to add cooling is to take off the CPU cover and put a table fan infront of it
