Hey @Quantum Torus There is no need to upgrade if your PC does what you want. for eg, I use Office 2000. Used Win XP until last Xmas.
Browsers are another kettle of fish. If you dont upgrade over time, they just dont work as well due to newer technolgies pushed out on the web. . Since I upgraded to Quantum its like night and day. Everything works properly. IT is new however, and still has a lot of bugs.
IT is without doubt the fastest browser Mozilla has ever released. Equal if not better to Chrome. It is the first release that you can have unlimited tabs open. IT manages memory very well. I have had no CPU thrashing at all, although I run a quad core. I would guess your mainboard, and hence CPU is old.