Will a PCI wifi card mess up my 660's airflow?



It may get in the way, but shouldn't have too much of an impact. I only have a dedicated graphics card, not a wifi chip, so I wouldn't know. I assume that the motherboard manufacturer wouldn't let it cause issues. My 270X has some USB connectors in the way, but my GPU stays perfectly cool.
 
As a rule no you wont see any increase in heat or loss of air flow. USB adapters (like for like) are just as good. Make sure you use a USB 2 or 3 slot for best perfomance. Down sides are it takes up a USB slot and it sticks out of the case so it can be knocked if front mounted.
 
USB wi-fi adapters are really good now.

For example, I got 7MB/second between the 2nd floor and basement where the wi-fi router is. My older router only got about 2.5MB/second but then that's the ROUTER as the bottleneck so a really good PCIe card might not be much better with that router.

My INTERNET connection is under 1MB/second. I also do local video streaming but I can handle all video (except BluRay copies which aren't compressed).

Some USB wi-fi adapters come with an extension cable which may help (may even be better than a wi-fi card that's blocked by the back of a desk).

I recommend something like THIS:
http://www.newegg.com/Product/Product.aspx?Item=N82E16833320178

If you want PCIe still, then THIS looks okay:
http://www.newegg.com/Product/Product.aspx?Item=N82E16833320074