Strange Ive played with a 8600GTS and overclocked it tons and it was ok. Must be a notebook thing, serves you right for trying to be bling
With ram and lag issues, try rebooting before you play the bigger maps like fools road and qinling.
Also try a program called enditall. It will close all the random **** on your machine not needed for running PR. Its not permanent, if any problem is caused then you reboot and its fine again, but it should just automatically know the programs you dont really need and you can alter the list manually if you like
Qinling takes about 2.2gig alltogether. If you have 2 gig of memory that will mean windows is going to have to shift at least 200meg of windows memory onto the hard drive, it wont do this immediately because windows is dull so your'll have to play a bit before it finally shifts the unneeded data onto the disk while you arent viewing windows.
It should be ok eventually, like I say reduce your overheads because it is a tight fit sometimes. If your disk light flickers during game, you know why your lagging as it should not come on after you have viewed that map grid allready (lag at game start is normal)
http://www.docsdownloads.com/Tier1/enditall.htm