I did some research for you. You do have a PCIe x16 slot and it appears that your computer will accept a standard ATX PSU. I've put together some recommendations as well:
If you are on a budget the 9500GT is about as low as you want to go. That said it will run the game at full graphic settings but will need at least a 350W PSU.
This Asus 9500 GT ($50) is probably the best price/performance card you can get in the 9500 series:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121267XFX has been building Nvidia cards much longer than ASUS and the XFX 9500 GT ($70) carries a lifetime warranty vs 3 years on the ASUS but the card itself is a little slower:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150332Sorry to say but PSU's aren't cheap. If you buy a cheap one you'll probably burn it and/or your video card up. This Seasonic 380W ($65) is from a reputable manufacturer and is 80+ certified:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151032If you have a little more money to burn on both the video card and the PSU you can step up to the 9600GT but it requires at least 425 Watts. The big advantage with the 9600 is more pixel shaders.
Like XFX and eVGA, BFG Tech has been making Nvidia cards forever and this is the fastest price/performance 9600GT I could find ($80):
http://www.newegg.com/Product/Product.aspx?Item=N82E16814143134Couple it with this 80+ certified Seasonic 500W PSU ($85) and you're good to go:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151056To install the PSU turn off the computer and unplug the power cord. Open the case and lay it on it's side. You need to unplug all the wires that come from the PSU to any part in your computer. There will be one plug (20 or 24 pin) to the motherboard, a four pin for the CPU plugged into the motherboard and connectors to things like hard drives, optical drives and the floppy drive if you have one. Once all the power connectors are unplugged, unscrew the PSU and pull it out.
Now reverse the process with the new PSU. Don't worry as each type of connector can only be used with it's appropriate device so you can't screw it up. Just make sure that everything that needs power has it. Take any unused connectors and bundle them up and tie wrap them.
As to installing the video card you have onboard video. First you'll need to uninstall the drivers for that card. They should be located in your control panal under add/remove programs. Just locate the video driver in the list and click uninstall.
Shut down your computer and install the new video card. It just slips into the slot. Make sure to screw it in to secure it.
Reboot your computer and enter the BIOS (usually delete or an F key during the post screen... check your computer documentation) and disable the onboard video. Save changes and continue to windows.
Once windows has rebooted it will tell you that it's found new hardware and launch the install hardware wizard. Close this and insert the CD that came with the video card. Select install drivers. Better yet would be to go to the manufacturer's web-site and download the newest video drivers for your card and install those instead. Any time you update drivers in the future you'll need to uninstall the current drivers before installing the new ones.
After all of that you can open the Nvidia control panal and set the cards global video settings. Don't be afraid to crank things up. You can always turn some things back down later if you're not happy with your frame rates.
Always remember not to wear shoes and to ground yourself to the computer's chassis (you just need to touch it) before working inside your computer.
