I can't speak to risers, I've been afraid to use them and refuse to pay current prices for them.  (I have a few 16x to 16x unpowered that do the job of separation just fine though)

As far as cards, the 7790 is under $100 typically and puts out about 225-235kh/sec, and by my calculations (based on the litecoin hardware page), the 7790's have the lowest power draw (over 3kh/sec/watt).  You could squeeze 4 of them and a motherboard/cpu comfortably on a 450-500 watt p/s.

The 270x is over 450kh/sec and they can be found for under $200.