In my opinion, it's better to stick to 2 cards per PC.
When you try to cram 3 or 4 cards in a PC -- you're not saving that much, really!
1000W power supplies are more expensive than 750W, which are more expensive than 500W. There's not even much of a "quantity discount" -- each card costs about $50 to power (assuming you're not going bottom-basement on the quality of your PSU)
And even if you go bottom basement -- you're talking $30 for one, $60 for two, or $100 for three. So it still holds.
Even if you were paying slightly more for 2 PCs, think about DIVERSIFICATION. With one tower, if that PSU goes south (OR your RAM, OR your hard drive, etc.) you're going to be getting 0 MH/s for a while. With 2 PCs, you'd at least get 50% of your usual MH rate.
One tower with 5 cards is "putting all your eggs in one basket" if I ever saw it!
And which would be easier to resell? Which one has more potential customers on Craigslist? Obviously the 2 "regular" PCs. More potential customers = more demand, which means you can get a higher price. So another +1 for my strategy.
You won't run into "Windows can only handle 3 GPU" issues if you stick to 2 cards per machine. Even just ONE card in a "micro ATX" single-PCI-E motherboard is not cost-prohibitive. You can get away with a smaller power supply, etc. I'm not advocating ONE per machine, but I wouldn't rule it out as being "not worth it" or something.
Motherboards with 3 or more PCI-Express slots are probably more expensive/rare than ones with 2 slots. That is true whether your source is online, ebay, or craigslist. There are simply MORE mobos out there with 2 slots.
And power consumption isn't much less (if it's less AT ALL) when you have 4 cards crammed into a full tower case with extender cables vs. having 2 PCs with 2 cards each.
I think it's less hassle and doesn't cost you any more (money OR power consumption) to have 2 cards per PC.
The cards are your real limiting factor. There are other factors as well (how much you can fit on one electric circuit at your house) It's easy to find a used barebones PC or build one.
Feel free to donate if you appreciate my advice