Yes, adding an SSD to an older PC often makes sense. The 'long' waits in desktop computing often come down to disk I/O -- for example booting Windows, launching Virtual Machines, compiling code.
Tom's Hardware recently had an article about just this: "Could An SSD Be The Best Upgrade For Your Old PC?", which I think you should read.
However, I would not invest a dime in a Pentium D (Intel Pentium 4 / "Netburst" derived architecture). That CPU is simply too slow.
Coincidentally Tom's also recently had a good article in which old and new CPU cores are normalized to the same frequency, to illustrate the Instructions Per Cycle differences. See the summary table here -- any P4 based PC is simply not worth investing in; a modern ~100$ Core i3 CPU will run circles around it. With both CPU's clocked at 3 GHz the P4 based CPU finishes their full benchmark suite in about 4 hours, while the modern "Sandy Bridge" based CPU only needs ~1½ hours.