less cores will likely be a bigger issue for SLI than alower clock. since the system can essentially asign a core to manage each GPU. its not quite that simple, but it does work out that way.
id go for the quad personally, it will be a little more futureproofed, better at ultitasking. and it will have the edge in threaded apps. and since more and mroe games are begning to use more than 2 cores, it will end up having better gameing performance too.
and like i said, overclock it!
lol, alot of people dont like to OC laptops, but i liek it more so than with a desktop. theres a great Program called 'SetFSB', i used it alot a while back. if you can gett your clockgen it will allow you to change FSB on the fly to change CPU frequency. heat is a bigger issue that in a desktop, but its still possible to get very good OCs.
all said though, if you dont plan on unning the latest games or any heavily threaded apps, the dual core will do fine.