Get an external drive and backup your data. Hard drives are cheap and they all eventually fail.
The case fans are cheap and easy to replace. Power supply is not very expensive and easy to replace.
If you like most of the hardware and have no need for anything better, it is usually simple and cheap to keep it running. At 6.5 years old, you could buy a motherboard/CPU/RAM deal off the Internet or MicroCenter as long as the motherboard form factor will fit into your case. Like getting a new engine in an old car, it only works if you like everything else about the old car.
First thing is knowing what your primary uses will be for the computer. I am very cheap and tend to run Linux on old computers until something significant fails, but I always keep backups of all of my data. It isn't "if" but "when" your hard drive/computer will fail.