For gaming at higher resolutions the GPU is the king, the CPU only makes a small difference. The difference between a CPU at 2.0ghz and at 2.8ghz at 1600x1200 is only a few frames per second, at lower resolutions it makes more difference but at much higher frame rates. In other words at 600x800 you might get 120fps at 2.0 and 145fps at 2.8, and the difference between 120 and 145 is insignificant at that high of a frame rate.
The 380w PS should be fine if you don't overclock, if you do want to OC you should get at least 450w. The 3800+ will OC to about the same as a 4200+ YMMV.
If you OC the 3800+ would be a wize choice, if not overclocking the 4200+ will be slightly faster in games, but not a very noticable difference.
And yes a 3800+ at stock speed is good for gaming