Upgrade or Buy?

We've had two PCs in our house for about five years now. While they were considered up to date at the time we bought them, they are now terribly outdated. Their memory is too low to do all the multi tasking we do, and forget about running multiple firewalls and spy ware to keep the kids from crashing our system while they play their games online. So husband and I have been contemplating whether it would be worth it to simply upgrade the Dell Memory on the systems (cheaper) or to just go on and buy a whole new set up (more expensive).