|Not laughing, but I would recommend updating the entire system (new build) rather than going through the expense of upgrading video card, power supply, Windows 7 (if it passes compatibility test), additional memory (unless you have 2GB or more for 32bit or 4GB or more for 64bit) since you will still have an old system and your CPU will be the bottleneck for gaming.|
Your PCIe slot if version 2.1 will handle the video card, Look here:
for motherboard specs.
For system specs look here:
You have to be a little bit crazy to keep you from going insane.