CPU upgrades tend to be overkill for most people (unless you're a pro with huge calculation needs it isn't really worth it).
GPU upgrades on the other hand will be the most important part for continued usefulness.
While the benefits of multiples cores in CPUs is mostly thwarted by the fact that parallelism is a pain to code, most devs haven't the technical skills for it and most software companies won't finance devs cost for a multi-core optimised software, the GPU side seems to be on a better path...
After openCL and CUDA (for scientific and Pro optimised software), and with the development of mobile platforms as the main future source of revenues for technological companies, a big push is being made for the mainstream adoption of heterogenous computation capabilities (Intel concurrents have just announced an association with such aims).
Meaning future software will tend to also use the GPU for more than just graphics, so if your computer has a good modern GPU, chances are it will become obsolete later than with the lowest GPU offered.
My advice get a SSD (the biggest performance upgrade right now), then get the strongest GPU you can (for future proofing), a consequent amount of RAM maxing it out even is a good idea (a lot of web browser optimisation tend to eat way too much RAM so 8GB should be a minimum if you don't want slowdowns from time to time), leave the CPU alone.
For the SSD I'm waiting on anandtech review of the fusion drive in the mac minis to see if it's worth it. But you can even set up your own fusion drive manually (I'm waiting for the BTO options prices to show, but personally I'm wishing for a 256GB SSD and a 3TB disk to set up my own fusion drive if performance of Apple system are good, otherwise I'll just keep apps & the system on the SSD and common data the mechanical drive).