Is it possible to just buy a powerful graphics card without buying a new PC?(for gaming)?

Unfortunately with a computer that old (sorry). You would either need to build a new one from scratch or buy a new computer. Here is why. You will need a processor that can keep up with the new video card. You will also need a power supply that can handle the video card as well. But back to the cpu, being 10 years old you would most likely need to change the motherboard to be able to accept a moderatly modern cpu capable of handling that video card. Another factor that comes into play as well is the memory (ram) wich may also need to be upgraded.