My computer says it has a PCI express x16 � can this slot work with a PCI express 2.0 video card?

what is the difference between PCI-e 2.0 x16 and PCI-e x16 (without the 2.0 desingation)? i have a roughly 4-5 yr old IBM thinkcentre m51 pc and want to select the correct pci -e card. i notice that newegg has to separate search categories for pci-e 2.0 x16 and pci-e x16. http://bit.ly/ji62QK