If your'e looking to upgrade go for a machine with a Nvidia Quadro FX card. These are cards specialised for technical use as apposed to gaming use. The latest mobile spec is Quadro FX 1600. CAD\3D Workstations typically spec Quadro FX 3500 but there are new cards just out (I think FX 4600,FX 5600)
Laptops to look at is Dell Precision M6300, Dell precision M90 (Very good deals at the moment) , HP 8710w and worth a look is the CAD2 IMAGINE Go Laptop Workstation.
If you have the money you can also look at Area-51 m9750 with dual graphics card option.
I recently bought an HP and it has handled sketchup models of substantial size very well.
While the dual core argument is true as far as sketchup is concerned bear in mind that your laptop will be a lot more responsive if you use other applications in conjunction with sketchup.
Hope that is of some help.