Introduction
The first question you need to ask is, what is the life expectancy of your existing system? Now don't listen to those that tell you that your system is outdated the second it comes out of the box. Everything loses value the second you buy it, that is how retail markups work. Cars can be useful long after they sink below their blue book value; the same is also true for that aging PC.
Now some systems are past their prime, five to ten years past and there isn't much use for them unless you like collecting nostalgic PCs or find new uses like turning them into routers or DHCP servers. I have one old Dual P90 server I acquired from NASA. It originally cost $114,000 in its time, now I only use it for demonstrating that old technology can still hold its own. Windows XP never had a harder time figuring out how to use that old hardware. If your system is in the gigahertz range and can use DDR, then you should be able to make it rise from the ashes.