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...
Yeah I have elec included in my rent so it's not a huge concern; however, if I could pull 550Mhash out of 115W it would seem doable to shove another 5830 in for some extra lower-profile mining. My impression is that 5830s pull about 180W.
Is $370 after rebate considered a good price on the locked ASUS DirectCU (minus OC)?