Join Date: Sep 2012
Location: Portland, Oregon
The Ivy Bridge Macs I've tried aren't that noticeably faster than the Core 2 Duo machines from 2009. I don't use Sketchup, but I do intermittently use Photoshop and Maya for work.
The biggest lag with modeling programs seems to be disk access. If you're working with a large file with a lot of vertices, you may notice some lag. Rather than getting a new Mac, you could probably try more RAM first, and look into getting an SSD if that doesn't help.
If you do decide to get a new Mac because it's just not cutting it with graphics, I'd opt for a Fusion or Solid State drive option over getting four cores or unreasonably priced RAM that you could buy elsewhere and install yourself for 1/3 of Apple's prices.
Also, if there's an option for discrete video opposed to built-in Intel HD Graphics 4000, you may want to consider it. The Intel HD 4000 does a surprisingly good job at accelerating video transcoding, but doesn't handle 3D rendering as well as the ATI Radeon 5770 or the NVIDIA GeForce GT 6XXM options available on some Macintosh models.