Is it time to replace my 6 year old desktop PC?

No, not too old, I suggest you get a better cpu, to begin with and then get some more ram. I say this because you have an older motherboard and the cpu for the socket has gone down in price. Such as,http://www.amazon.com/Processor-Athlon-Energy-Efficient-Socket/dp/B001C25672 . And for the ram, 2gb of ram is the minimum for windows 7 so if you get 4 gb you should be good and you should see a difference because it is not always capped at 2gb. Lastly, you should either get a new hard drive or clean out the old ones you have because you should have 15% of it free so it can have space to move files around, so uninstall any programs you don't use and any file you don't need.