I weighed the pros and cons and decided against stocking up with 5xxx series cards because a) they're hard to find and are somewhat overvalued by merchants aware of the bitcoin craze, and b) 6xxx series cards are cooler and use less power while putting out comparable Mhash/sec rates so c) you'll have a use for them either to resell to a gamer or use for yourself when the difficulty becomes so high that your minings returns suck.
So, I filled my new system with three 6970's, running them off a gold standard PSU, Corsair Professional Series, 850W. Do NOT skimp on that part of your system. I plan to run them in Xfire when my mining days are done, and it's sufficient for two or three cards if you don't overclock the cpu.
In fact, get the cheapest CPU possible since mining doesn't need it, I put an Athlon II 260 for like 50 bucks into my Asus Crosshair V AM3+ system since I'll replace it with a Bulldozer processor in probably August or September when the difficulty will be so high that it'll be somewhat difficult to mine effectively.