+1 for upgrading your current machine.
I'm using a 2008 MBP 15" (2.4Ghz, 256GB SSD, 8GB RAM), and it fulfills my needs (basic office stuff, Xcode, Matlab, Latex every now and then) just fine. Sure, some things could be a little snappier (compared to my i5 gaming machine or my i7 notebook at work), but that's nothing I'd spend thousands of $ on.
If you don't have to care about money and really want to get a new computer then sure, get it, but performance-wise there is no real need to do so (with regard to your needs).