It's Corsair's low-end PSU. Corsair made the CX series with the intention of creating a good budget, yet good, PSU for basic systems so they don't have to pay a premium of a high powered PSU when they don't need performance. But you have a gaming rig which is high performance, so I don't recommend the CX models. However, the 750W CX is more than enough to power your system and the HD 7870 XT. It will be fine for CrossFireX HD 7870 XT as well.
Some quality PSU to look at are: Antec EarthWatts, Antec High Current Gamer, Antec VP, Cooler Master Silent Pro, Corsair TX/HX/AX, Enermax Platimax, Lepa G, NZXT Hale90, PC Power & Cooling (all), Rosewill Capstone, Rosewill Hive, Seasonic (all), Silverstone Strider Gold, Silverstone Strider Plus, Super Flower (all), Thermaltake Toughpower, XFX (all).