As others have mentioned, even if it's not PCIe 2.0, it can still work.
If my understanding is correct, PCIe 2.0 (how the video card "communicates" with the motherboard) has a faster transfer rate than its predecessor, but PCIe 2.0 cards will work with previous versions of PCIe.
To see which bus your motherboard is using, try looking up your computer's model on Dell's website. It'll usually have a list of hardware. Unfortunately, sometimes they use different hardware with the same model, so I'm not sure you'll get a definitive answer form there. Do you still have the order paperwork (and/or e-mails) from when you got the computer?