You should request a mac pro so you can gut it for some better hardware.
In all seriousness though, it should be fine. Heres some benchmarks:http://www.notebookcheck.net/AMD-Radeon-HD-6770M.43955.0.html
Note that in blackops you can easily get 50-60fps on high, 35 on ultra. You can also expect slightly better performance than many of the benchmarks since it's a mobile gpu most of these benchmarks are run on laptops and the imac has a desktop cpu, so you're faster in that respect.