not sure what you're going to say is a decent card is but i use a geforce 8800gt with an intel mac 2.66 quad.. 5g ram...
truth is, i don't notice much of a sketchup performance boost with my desktop over my laptop (g4 powerbook... 1.33 PPC chip.. 1.5g ram..)... it's just that on the laptop, i pretty much have to close all apps except sketchup to get it to run good so in the case of multitasking, multi cores are a no brainer.. (in contrast, i do a lot of image toning/management/editing with ~80mb files and there are incredibly noticeable performance differences between the two machines while using apps such as photoshop...)
.. most sketchup operations aren't very demanding and i only notice major enhancements with the faster chip for certain things -- complex follow me deals, soap skin bubble, joint push pull, copy/move etc.. those are the instances where i wish for multicore support because i do a lot of complex curve type of drawings which i think i could simply coast through if i weren't relying on one core..
strictly speaking sketchup however, a quad is going to offer little to no performance enhancement.. to say otherwise is misleading.. my quad can deal with higher entity drawings better than the laptop but that's due to the better card.. still, that difference isn't highly noticeable and once i get to around 25,000 pieces, the quad will start to glitch.. the problem is in the programing and you can't just throw money at it to deal with it.. instead of doing the my machine is bigger than yours argument and discussing minimal gains after a couple thousands of dollars worth of upgrades - it would make more sense to get the programming right first...