It will work in pretty much any mobo that has at least one PCIe slot. It is a PCIe 2 card so it would make sense to get a board w/ PCIe 2 slots although it does not make much if any difference now. Where the decision lies is do you ever want to be able to use more than one video card. At least until X58 mobo's come out you would need to buy a board with a Nvidia chipset that has muliple PCIe slots and supports SLI. If you are using a current Intel chipset you would be limited to one card but you could run an X2 card like the 9800 gX2.
Same thing. If you only want one video card, your best bet is a P45 motherboard. The GA-EP45-DS3R and Asus P5Q Pro are my favorites. They have the same features, except that the P5Q Pro has 8 SATA ports instead of 6. That's useful to some people but not many. I use 7 ports right now, but I don't know anybody else who does
The GA-EP45-DS3L and the P5Q SE are also good and a bit cheaper, but IMO you lose too many features with them and not save that much.