Help me chose a power supply

Lets take a big step back and rebalanced this system. You are getting an i7 960, 12GB of ram, and a Rampage III board, but sticking with a 2400XT? What are you doing with this system?

As to your original question, do you intend to crossfire 5870s in the future? If so there i recommend a good 750W unit with 60A on the 12V rails like the Corsair 750TX or the Antec TP-750, if you only intend to do a single 5870 a good 500-550W unit like the Seasonic S12II 520, Antec TP-550 or Corsair 550VX would all be fine. Depending on where you are buying from and what case you are getting you might be able to find a combo on one of those units to make it a really good deal.

While you didn't list the number of HDDs or CD/DVDRWs, you should do fine with an 80+certified 550W or 650W PSU. I've been very happy with my Corsair CMPSU-650TX for the last 18 months. Corsair also makes modular versions, where you only install the power leads you actually need - helps with cable management inside the case.

Here you have some good options for you:

ANTEC EarthWatts 650W (by Delta Electronics)
ANTEC TruePower New 550W (by Seasonic) (modular)
CORSAIR VX550 550W (by CWT)
ENERMAX PRO82+ (EPR525AWT II) 525W
ENERMAX MODU82+ (EMD525AWT II) 525W (modular)
SEASONIC S12II Bronze (SS-520GB) 520W
THERMALTAKE ThoghPower XT (TPX-575M) 575W (by CWT) (modular)
OCZ Fatal1ty 550W (by Sirtec) (modular)
COOLER MASTER Silent Pro M (RS-600-AMBA-D3) 600W (by Enhance Electronics) (modular)
GIGABYTE Odin GT (GE-S550A-D1) 550W (by CWT) (modular)

I will use it as a gaming computer. Sorry that I forgot to say that I will have 2 HDDs and 2 CD/DVDRWs.

So I'll be good with a 750w PSU?
Edit: No I won't crossfire the cards.