Yes, you need to be concerned with signal degradation at 15 - 20 feet. That is beyond the limit of the USB specification, so while you may be able to purchase a cable that is the required length, don't be surprised if it doesn't bear the official USB logo. If you need to go that far, get a USB cable that has the active signal booster.
Video is in a similar boat. While you don't necessarily need to get a signal booster for that length, there is no error correction on DVI signaling (including HDMI), so if you get a cheap cable you will get signal drop-out or corruption.
Do not run any of your cables past power strips, wall transformers, electrical service panels, or other areas of high interference, for best results.
I usually buy my high quality cabling from this outfit. They have pretty reasonable prices:
http://www.pchcables.com/