Are there any requirements for a computer when it comes to being compatible with a graphics card or can i just use my old computer and a graphics card which is much newer (if i bought a good enough PSU)?
More aboutgraphics card compatibility
The type of slot is very important. The new GPU's need a PCI-E 2 or 1 slot. If you have an old pc it could not have that. ( only pci maybe )
Depends on your specs. GPUs need to be compatible with motherboard and PSU. What are your specs and what GPU are you planing to buy ?
It also needs to be "compatible" with your CPU. I was just in a thread where someone wanted to spend $200 to get a GPU. He was still using an old P4 CPU however. He wouldn't get the frame rates he was thinking he'd get if he used such a slow CPU.