Need Help With Graphics Cards

Hi all. I'm looking to buy a new graphics card for my computer. I have never purchased a graphics card before and am running on the card the system came with. In fact, all of the computer is original hardware, nothing purchased extra. I happen to be a teenage kid 90% unexperienced with these kinds of things. I am having trouble finding out if my computer is running off PCI, PCI-E, or AGP. I am a major World of Warcraft player, and I am tired of running of 15-30 framerates and getting constant screen lag, even with all settings on low. My latency never goes above 300, and besides framerates everything runs pretty smoothly.

I am currently running off a 2007 Windows Vista, HP Pavilion desktop. My main problem is as i said above, whether my computer is PCI, PCI-E, or AGP. Below is what my system is from what a program told me.

Processor: Intel Core 2 Duo E4300 @ 1.80 GHz
Model type: F

Mainboard Information
Manufacturer:ASUSTek Computer INC.
Model-LEONITE
Chipset:i945P/PL/G/GZ
Southbridge:82801GB (ICH7/R)

Memory Information
Type-DDR2 Channels-Duo
Size-1014 MBytes
DRAM Frequency floats around 266.7 MHz.

Graphics Card info-Intel(R) 82945G Express Chipset Family 256 MB

Thank you for your time.