They will work, but the motherboard will not utilize the 2.0 technology unless specified. I am pretty sure boards such as the 680i, 780i, and x38 chipsets support 2.0. P35/31 and lower end chipset models do not.
Yes. Read this article for more info. It would be best, I suppose, for you to couple a compatible card with a compatible motherboard. It's definitely not necessary, though.
Q4: It sounds as if both speed grades are supported in the PCIe 2.0 specification?
A4: The PCIe Base 2.0 specification supports both 2.5GT/s and 5GT/s signaling rates, in order to retain backward compatibility with existing PCIe 1.0 and 1.1 systems. Aside from the faster bit rate, there are a number of improvements in this new specification that allow greater flexibility and reliability in designing PCIe links. For example, the interconnect can be dynamically managed for platform power and performance considerations through software controls. Another significant RAS feature is the inclusion of new controls to allow a PCIe link to continue to function even when some lanes become non-operational.
Q5: Then PCIe 2.0 must be backward compatible with PCIe 1.1 and 1.0?
A5: Yes. The PCIe Base 2.0 specification supports both the 2.5GT/s and 5GT/s signaling technologies. A device designed to the PCIe Base 2.0 specification may support 2.5GT/s, 5GT/s or both. However, a device designed to operate specifically at 5GT/s must also support 2.5GT/s signaling. The PCIe Base specification covers chip-to-chip topologies on the system board. For I/O extensibility across PCIe connectors, the Card Electromechanical (CEM) and ExpressModule™ specifications will also need to be updated, but this work will not impact mechanical compatibility of the slots, cards or modules. Currently, the PCI-SIG is defining the PCIe CEM 2.0 specification which has been released to members for review at v0.5. There are currently no plans to adapt the PCIe Mini CEM specification for the faster bit rate as the market need has not yet materialized.
Q6: What other new features are introduced in the PCIe 2.0 specification?
A6: The most predominant new feature in PCIe 2.0 is 5GT/s speed, which includes new mechanisms for software control of link speed, reporting of speed and width changes, and control of loopback. Other new features include:
PCI compatibility using the established PCI software programming models, thus facilitating a smooth transition to new hardware while allowing software to evolve to take advantage of PCI Express features
Enhanced Completion Timeout Control, which includes required and optional aspects, reduces false timeouts and increases the ability to ‘tune’ the timeouts
Function Level Reset and Access Control Services, giving enhanced robustness and support of certain IOV features (optional)
Slot Power Limit Changes to allow for higher powered slots, which support the newer, high-performance graphics cards. This new feature works in tandem with the 300W Card Electro-mechanical specification
Speed Signaling Controls to enable software to determine whether a device can operate at a specific signaling rate, which can be used to reduce power consumption, as well as provide gross level I/O to memory