Should I get the new MacBook?
For the quotidian tasks you mention, your computer is adequate, although I would immediately increase RAM to 4GB, a rather trivial expense.

Mathematica &amp; MATLAB, and other CPU-heavy apps such as Strata Design 3DCX or FinalCut Pro X, have the ability to run multiple cores in parallel. In most cases, ordinary apps will run only one core of the CPU. Other apps, such as Photoshop will take advantage of multiple cores for a subset of its available operations which can benefit from parallel processing.

Mathematica, in particular, does not run most of its calculations on multiple cores, but offers operations which provide parallel computation.

If the MacBook is in OK shape, (i.e., you treated it decently,) the best options would be to increase the RAM, and if you choose to do so, install an internal SSD. At 6 years of age, the internal HD is closing in on its eventual demise.