I hope I can help!
I believe you are correct. Unfortunately, your PC does not support PCI Express 2.0, so it will not function with your computer. If you can return the video card, I would do so, depending on where you bought it from, many manufacture's will allow you to "RMA" the video card. If you would like information on how to do that, I can probably help you, but would need to know who you bought it from. Probably the easiest course of action would just be to contact them directly though.
Now, as to which card you can use, you basically need a card that is PCI Express x 16. This is a slightly older format, so you will have some trouble finding a large variety of cards. For the most part, the cards I will give you links to should be supported by your computer.
Newegg is a great place, but it looks like most of their PCI x 16 cards are out of stock right now. You can also try Tiger Direct, Frys, and if you have to, Best Buy.
I hope that helps!
By the way, this is a link to the specs of your computer if you would like it.
Link