For gaming, the maximum performance increase is noticeable in GPU overclocking. However, if you are using a slow processor which might bottleneck the card, then overclocking the processor might improve gaming performance.
Also keep an eyes for these generalized signs of a bad overclock-
- CPU & RAM - Computer freezes or BSODs like- IRQ_LESS THAN ZERO/MEMORY MANAGEMENT/...etc.
- GPU Core/Shader - Computer freezes, Screen goes off (No Display), System restarts when running 3D Applications or Games.
- GPU Memory - Garbled display, Poor textures, Anomalies when running 3D Applications of Games.