A single 4850 will draw about 150W max, less in normal gaming:http://forums.atomicmpc.com.au/index.php?showtopic=264
GTX 260 is just a bit more, maybe 175W
So yes, a PSU actually capable of 350W continuous load would be safe enough. We would never recommend such but in the world of pre-builts they like to cut things close and save every dollar they can. That said their PSUs usually do operate within ATX specs, so no worries.