Depends on what your power rates are - I'm currently locked into quite an expensive contract for the next couple of months and my three reference cards are sat mining at 950Mhz & 0.95V. Gets about 550Mhash per card and uses really very little power (about 115W per card).

To be honest, I'd just pick up the cheapest that you can get decent RMA support with - the 7970s haven't seemed fragile so far but we all know what mining does to these sorts of cards long-term...