The two processors Seem to be:
and the differences are more than just clock speed - the faster one also has faster memory bandwidth, along with plain old 'more features' such as VT-d Virtualization for directed IO, Execute disable, Quicksync video, wireless display, mywifi, 4G wimax... but what it means for those things to have built in support in the CPU, I don't know.
I'd predict that the memory bandwidth, which is about 20% higher in the faster chip, would have more of an observable effect than the 10% faster CPU, but that unless you have an intensive use planned the difference wont be worth much worrying about.
Far better to look at an SSD instead of a normal hard disk, that will provide a big shift in the feel of how responsive everything is - the hard disk is the biggest bottleneck in normal computers these days. (Where a hard disk can shift 20-40Mb/s sustained, an SSD can shift 100-200Mb/s sustained. Where a hard disk can handle 100 operations per second, an SSD can handle many hundreds or a few thousands).
For which types of applications will the different graphics cards have an observable effect in performance?
- Graphics heavy games, racing and running around shooting, flying and the like (not cards, dice, board, web/flash games, etc).
- Graphics heavy apps like architecture modelling, 3D scene rendering, Pixar style film rendering.
- Currently niche apps which make use of the graphics card as a spare processor - at the moment this means things like distributed computing project SETI@Home, and PowerDirector 7 video encoding software. However, there is a push in the industry to make this more widespread, but that's probably still too far away from every day uses to bother about for another year or three.
My vote is that unless you have a particular intensive workkload or unusual use which you haven't mentioned, the 2.0Ghz will be fine, and if you can spare the money then see if you can find a machine with a good SSD to compare, and consider one as an upgrade, for an everyday snappiness boost. (Apple supplied, or aftermarket).