An SSD could be transferred and used again if you built/bought a new PC at a later date, so could be a worthwhile investment.
An SSD would speed up boot times, programs would open more quickly etc, but if you want to play games more modern than GTA San Andreas I'd recommend a new PC.
I don't see machines with 5GB RAM very often, or such a combination of hard drives, looks like the previous owner did a couple of upgrades as and when required!
If you have a spare licence key sitting around for a 64 bit operating system then install it, but I wouldn't bother going out to buy a new OS for it!