what is PCIe 2.0

will my PCIe x16 MB run a 8800GT that is PCIe 2.0? What is the difference between x16 and 2.0?

2.0 has more bandwidth than 1.0 x16, they are essentially x32. They are backward and forward compatible.

Try Wikipedia next time (they are quoting Anandtech here)
The transition to PCIe 2.0 won't be anything like the move from AGP to PCIe. The cards and motherboards are backwards and forwards compatible. PCIe 1.0 and 1.1 compliant cards can be plugged into a PCIe 2.0 motherboard, and PCIe 2.0 cards can be plugged into older motherboards. This leaves us with zero impact on the consumer due to PCIe 2.0, in more ways than one.

Pci-e 2.0 is just an upgrade for future cards that need more bandwidth. X16 is the slots physical width and bandwidth measure. Current PCI-E 1.1 has a bandwidth of 250 MB/S per lane while PCI-E 2.0 has 500 MB/s. This really has no impact on cards that run at X16 lanes, but it will help cards running at X8. Both MB and the card must be PCI-E 2.0 for it to run at the increased speeds.