Should I get a new PC?

You should be fine with just replacing your RAM and GPU; I still have a 2GB memory but an NVIDIA 8800 GTS. All of the games I've thrown at it so far (Crysis/Far Cry on High which ran fairly well) have worked fine and there's hardly any load time lag. If you have money to throw around then go ahead, those specs should last you a while. I have a dual core 2.42GHz processor, and it still works like a charm. As for the motherboard, you should be fine.