Can I use a PCIe 2.0 card in an old PCIe slot?

[Aux10]

Ok so I was working on the innards of Boinc Server 2 (one of my back up servers) to keep it up to speed during the prime grid challenge and I noticed something. Last time I looked in there I had no idea how to tell the different slots apart. So I looked in there and despite being an old Dell Dimension 4700 I saw everyone's favorite little blue slot. I looked up the Manuel online to find out that it was indeed a PCIe x16! YAY! Now my Nvidia 210 card which I will be replacing soon is a PCIe 2.0 card. So here's my question. Now I know that regular PCIe cards will work in PCIe 2.0 slots but will a PCIe 2.0 card work in a regular PCIe slot? I sure hope so cause I'd love server 2 to get promoted from backup status to a primary system. (I say back up because I don't see the point in paying the electric bill for such a low producing system 24/7.) Oh! just one last question, which linux disto will be the most efficient credit wise for CUDA. It's a Ubuntu box but the last time I tried CUDA on Ubuntu (Thats no typo, despite what every ones been saying I really did manage to put CUDA on a Ubuntu box) the credit output us not living up to the card potential.