Poorly, I believe
Absolutely.
The MacBook with 9400M scores about 2100 in 3D Mark06.
The last gen iMac with Radeon 2600 Pro (or underclocked XT) scores
something like 4300.
The new entry level iMacs are surely faster than the MacBook because of higher clocked
CPUs, but will be much slower than the Radeon 2600 in gfx tasks. The new chip is
comparable to the old Radeon 2400 that was used in older entry level iMacs.
The only read advantage is that Apple uses the nvidia chips for some hw optimizations for
video playback. So the new iMacs probably play HD videos at lower cpu usage than the older models.
Christian