I agree 100% with vehelpfulguy! upgrading the CPU within the limits of the motherboard gives minimal performance difference where as a memory upgrade is far better. A new PC will give the most economical benefit.
Yep, at that age, you'd be throwing good money after bad. Your PC is already obsolete, and the older it gets, the harder it will be to find replacement components for it. Much better to put that replace / replacement money into a new PC - you'll be very glad you did.