Well, the good news is that it is end of Nov already so you won't have to wait too long to get a new computer.
If you can't wait, you can try swapping out the motherboard and CPU with a newer design that has multiple cores. This is fairly complex however, as you will also need to re-install the operating system. You might also need a new power supply if you old one does not have all the connectors....
I'll wait for the new PC if I were you.
Yep. The "upgrade" path on that is pretty much building a whole new PC inside the old box. Can't be 100% certain, but I'd guess once you start buying some new parts (MB/CPU) you'll find that you need to replace just about everything else, too. And at that point, you may as well buy a shiny new box to put it all in, while you're at it, so you can keep all the old parts in the old box, running ... whatever it is you run now.
So if you're comfortable with building your own, then wait and buy everything at once so you can be sure it all matches and will work together. And if not, wait till you can buy a pre-made from ... whoever you like to buy from.