I have a 1.5 intel centrino laptop with mobile radeon 9700 graphic card (64 MB vid memory) and 512 MB ram. SH3 (GWX) is running just ok, except of many explosions in the same time. No info on SH4, because not really interested in that sim. The advantage of centrino is, you can tone down the core voltage with some free tools (default 1,340 V, my laptop running on 0,940V), which means a major reduction of heat and power conspumption from the battery. Under normal work, the temperature never exceeds 52 deg C, and it is not necessary to use a cooling fan at all
So I would also recommend Intel and not AMD, as they are getting much warmer, like Canovaro allready said.
cheers