First do some research and find out how much it would roughly cost to buy a PSU, hard drive, motherboard and CPU. You've already got some pretty good parts with the video card and the RAM - it would be a shame to buy a new PC when you've already got them.

Like I said, see how much it would cost to replace the CPU, hard drive, motherboard and PSU. I'd give it €350 - €500. But that could be well off. When you know how much, you'd be able to make a good decision on whether to get a new PC or not.