It totally depends on the way the games are programmed, and in some rare cases the way the hardware is designed (IE Sega Saturn; a nightmare to overclock, and the Sega CD where the slave 68HC000 CPU is used to control CD-ROM, BIOS, and Genesis communication). in the case of the DreamX, only some cut scenes were affected with a physical speed increase. in cases such as this the CPU is throttling the video at a steady 60 FPS as coded.
I can send you somebefore and after videos where overclocking makes some games playable! metal Slug on the Neo Geo is one such title.
I always install a clock select switch that allows for stock gameplay, this resolves incompatability issues.
The saturn still gives me chills, I hate that hardware.