The PC freezer posted will be easy to upgrade. For now the GPU will be the weak point. If you were to decide to go crazy on a 1000$ GPU, the CPU would bottleneck and you'd need something faster. If you only go for something mid end the CPU will be OK. Plus you can get a nice OC out of it
As for your current PC. It's too shit to be worth spending any money on it. You could get some PCI video card, but it would be slow, painful and a huge waste of money
once upon a midnight dreary, while i pron surfed, weak and weary, over many a strange and spurious site of ' hot xxx galore'. While i clicked my fav'rite bookmark, suddenly there came a warning, and my heart was filled with mourning, mourning for my dear amour, " 'Tis not possible!", i muttered, " give me back my free hardcore!"..... quoth the server, 404.