Hello everyone. Thanks in advance for your replies.
I would like to replace the graphics card that was included with my computer with the highest graphic card that I can buy. However, I do not know what type of card I can buy. The graphics card that was included in my computer is an Nvidia GeForce 8500 GT graphics card with 256MB dedicated video memory. I know that I will be buying a card with a PCI slot, but other than that and the fact that I need to take into account my power supply (I don't know the wattage), I do not have the foggiest idea of what type of card I can buy. I consider myself a novice at replacing and making improvements to my computer, however, I have never replaced a graphics card and would really appreciate everyone's help.
The HP m8400f has a 300w power supply. Your best bet is to install a HD4670 graphic card which runs on the power provided by the PCIex16 socket. You just have to unplug the 8500GT from the socket and install the HD4670. Uninstall the drivers for the 8500GT, replace the card and download the drivers for the ATI HD4670. To get a card with better performance then the HD4670 you'd probably have to replace your power supply.
Here's a graphic card hierarchy chart. You can see where your 8500GT is and how much of an improvement a HD4670 would be. Also you can check to see if it's worth it to you to replace the power supply and get an even more powerful card.
Thank you very much dirtmountain for your advise. I am going to check out the two cards that you included ot your reply and see which one is better. A couple of months ago I thought I had bought a really good computer, but now that I get more involve in games I see that I might need to make a lot more improvements. I saw the graphics chart, but I don't understand how much difference buying the new cards will make; I guess I don't understand the chart you enclosed link that has the chart. However, I do see that unfortunately I can only go so far if I do not replace my power supply with something higher. I think as I start replacing so many computer parts my cost would be a lot higher than simply buying a dedicated gaming system, such as a PS3 or XBOX.
Finally, I also want to thank you for attaching the link to the hardware temp. sensor.