Basically, the 5800 may not be powerful enough to run N-Gage games, at least not in 3D at the phone's full resolution.
The 5800 has the same speed processor as most current N-Gage phones like the N81, 5320 etc, but the 5800 has a MUCH larger screen resolution (640x360 instead of 320x240), and much higher res means the processor has to work much harder to render 3D. For example Bounce on the 5800 is a lot jerkier than Bounce on current N-Gage phones.
2D games shouldn't be a problem, and it's a pretty fast phone in other ways, but full screen 3D games seem to be its weak point.