I need a 20ft DVI cable. Is there a good cable that is not too expensive (preferable not much above $40). I read that DVI can have problems with long lenghts, but that a good well-shielded cable should be fine. Grateful for any help. (Hope this is the right forum too.)

The DVI spec says that cable must be able to handle 5 meter lengths (16 ft) just fine. So finding one that works at 20 feet doesn't sound too hard to accomplish, since most cards are a bit overpowered on the output anyway. Any 20ft DVI cable should do. No need for extra shielding or anything absurd.

Look for a DVI-D cable to ensure a digital connection. DVI-I is an integrated cable which can handle the analog and digital signals, but at longer lengths, if the display can't read the digital connection, it can switch to the analog one automatically, so you won't get the best picture. A DVI-D cable is digital only, eliminating the possibility of it falling back to analog. So if it works, you know it's working at it's best.

Also look for a cable that's "Dual-Link", as these will allow the signal to go further than a "Single-Link" cable.

Never get a DVI-A cable. There's no point.

Thanks for the advice!