A single good GPU like the ATI 5750/GTX260 will work fine. As for RAM, again, depending on what you are doing, 8GB, else 4GB should work. For example, if working with many RAW images at once in Photoshop, I highly recommend 8GB. Mind listing out the rest of the specs?
Just curious, why SketchUp? If you can get AutoCAD I assume you can get Inventor/SolidWorks? Never got the hang of SketchUp, imo, the other CAD software are far ahead of SketchUp.
I forgot about the GPU- AutoCAD and Photoshop don't require much in the way of video cards. I don't personally know about sketchup but I doubt if it makes use of a fancy video card either.
You can run AutoCAD on any PC, no matter what video card it has, and you can't tell much difference from one to another. Revit is a little different but it still doesn't make that much use of the video card. Most people put at least the basic workstation video card into their CAD machines if for no other reason than to get good drivers that are made to work with autocad. This especially helps if you get a 64bit OS.
Now gaming is a different story, some games want all the GPU they can get.
I've never used inventor // solidworks but I will look into them... Sketchup is perfect for what I do currently though. I'm a landscape design student and Sketchup is a perfect conceptual tool that you can throw a model together real fast and than draw in by hand what you like, it just gives you the general shapes of things and perspectives to trace, etc.
^ If you are a student, and have a .edu Email, you can get Student Editions (13 month license, you'll just upgrade to the next version every year to continue using it for free) of all/most Autodesk products here: http://students.autodesk.com/
Imo, if you are doing landscape,etc you are better off teaching yourself AutoCAD Architecture, Civ 3D. 3DS Max, and/or Maya as these are much more widely used than SketchUp. Inventor and SolidWorks are more for designing parts for machines,etc.
If you are using solidworks, keep in mind that SW10 x64 needs more ram than SW10x86. My laptop is a core 2duo with 2 gigs ram from 2008 with an intel integrated graphics chip. When I installed SW64, it was very slow. When I switched to 32 bit solidworks, the performance improved significantly.
That being said, I just build a desktop with a Phenom II x4 965 and 4 gigs of ram and I have no problems with a 400 component assembly on x64. Now Solidworks Simulation is a different story. Im also a Mech. E, so I do pre-ansys FEA quite often. If you are just 2D sketching, you will definitely be ok.