It is entirely possible to run a build with two 6850s on a good 550W PSU.http://www.anandtech.com/show/4061/amds-radeon-hd-6970-radeon-hd-6950/24
Idle = 170W Load - Crysis = 378 Load - Furmark = 423W
Shows a system with an i7-920 (more power hungry than an i7-2600K) using 423W under Furmark (which is an unrealistic load). Even if this measurement wasn't from the wall, which I think it probably was then that would only stress a 550W PSU to ~77%, which if the PSU is high quality wouldn't even cause it to break sweat. Obviously feel free to choose a higher wattage unit if you don't feel comfortable pushing your PSU to ~70% on fairly rare maximum loads. Or if you want more upgrade options. Or if a high quality 650W unit is better priced than 550W units.
If this is a gaming build then you should get the i5-2500K (if it is cheaper which it usually is) as the i7 has no advantages over the i5 in gaming.