I need to order longer cables for my flat panel monitors and was going to get them through monoprice.com. They have DVI-D and DVI-I cables. I assume I would need DVI-I since the video card also outputs analog, but I also assume the DVI-D cable would work as well.

Which is usually used on monitors? I would rather buy the most common one. I assume it's DVI-I.

DVI-I (dual analog and digital) cables will have the benefit that you could later attach a DVI -> VGA adapter and extract the analog signal. DVI-D (digital-only) obviously won't support that. According to this site, you can plug a DVI-D cable into a DVI-I connector, so I'd make the decision based mostly on the price.

OK, cool. DVI-I it is then. I just wanted to get the most common ones in case I sell these monitors with the cables at some point. The cables I have are only 3 ft. for some reason.

Just make sure that your TV / monitor will except a DVI-I cable. It probably will but there is a chance that it has a DVI-D socket in which case it will not work with a DVI-I cable.