Graphics Card Installation Help PLEASE

[Waterfall]

I just bought nVIDIA GeForce FX5200 for my PC. I opened up my computer and took the other card out no problem. The problem is that the new card is much larger than the old one and doesnt fit in the slot. The guys at Best Buy told me this thing would fit in my computer. Did I buy the wrong size card? I didnt even know there was different ones. If I have to go go return this bad boy what should I be looking for in a new one? I just want to be able to pllay a few computer games Im not looking to spend much money. Any help or input would be appreciated guys I am lost here.

It says on the side of the box I need a Motherboard with an AGP 2.0 Slot. I am guessing I dont have one. Do they have cards to fit my computer, if so what are they called? Do I need to return this one or is there still a way to install it?