Is there a way to mine with many graphics cards without having a PC for each one (or two)? I have a serious stash of radeon cards, some great, some fairly crap. I figure with them all running it all adds up.

Problem is, running a PC for each card will get expensive, so is there any known rig or way to mine using a bunch of cards and NOT have them all plugged into a PC of their own?
I have found that the cheapest way to hold many cards is to put them in old motherboards.  I have been buying motherboards off Ebay for $45, getting a $5 processor, a $100 PSU and connecting 4 cards per rig.  I currently have 6 of these rigs running and they are great.  As for heating I have built custom cases for my cards so they don't overheat.  Using this method I've brought the cost to run each card down to about $50.  In other words, it will cost about $50 per card to put it into a computer if you build your own off Ebay.

Another option, which is definitely more expensive but probably the type of thing you were hoping to find is a PCI Express Switch.  These things exists and are how big companies can connect 20+ GPUs to a single computer, but they are prohibitively expensive in most cases.  

Below is an example of my mining tower, if you would like more information on exactly what I buy and the economics behind it please don't hesitate to leave me a message.