General rules about spec-ing any computer....
For any performance sensitive application, and often just for general use, you always want to get the fastest processor, and the maximum RAM that you can install (or at least afford to get). Usually, you can upgrade the RAM later, unless it is already maxed (or buy an Air). But Macs don't upgrade CPU processors easily, if at all.
For disk based apps, you need the fastest hard drive that you can get. A 7200 RPM drive is faster then a 5400 RPM drive. A SSD is faster than a 7200 RPM drive. In most cases, the hard drive can be upgraded later.
Gaming (and some heavy-duty graphics and video apps) adds the need for the best video card that you can get. The only Mac that can upgrade the graphics card after it is built is the Mac Pro.
Individual CPU and graphics card models change almost monthly, hard drives somewhat less often. But the general rules still apply.
