PCI-E 2.1 card in PCI-E 2.0 slot?

Is this possible, or will the power fall short? Looking to replace my video card.

I assume PCI-E 3.0 cards won't work in a 2.0 slot either.

PCIe should be forwards and backwards compatible, with you getting the features/bandwidth of the highest common denominator between chipset and card. I'm aware of one incompatibility, which my brother fell foul of.

Usually with hardware if they're going to break compatibility, they change the physical slot so you can't plug it in, for example with RAM.

Wait, I could put a PCI-E 3.0 card in a 2.0 slot? Really? I'd believe the other way around, but I just want to make sure here.

I currently have a 2.1 card (Radeon 6970) in a 2.0 motherboard (Intel x38). No problems.

PCIe is fully back- and forward-compatible. It just uses the least common denominator of features. If you buy more advanced cards, you can take them with you to later motherboards and have them magically improve their bus connections. Version 3 cards work in version 1 slots, and vice versa, with the caveat that there was that one bug that Scratched is pointing out, from six or seven years back. That was a bug, and most manufacturers fixed it.

Video cards don't generally need that much bandwidth. An x4 PCIe 2 slot is fine for just about everything, only a tiny bit slower than x16. So it's not likely that pulling a video card forward into a later system will make it speed up in any measurable way, even if it is using a faster bus connection.

Thanks very much for the clarifications, guys, I appreciate it.