In my opinion, as long as the AGP and PCI slots are locked and the CPU remains at stock voltage, there is not very much risk involved with overclocking apart from instability, and possibly frying another part, such as memory or motherboard, but that's really unlikely.
If you don't know much about how computers work and are not good with troubleshooting them I would advise against it, you may need to reset the CMOS if your system becomes unstable.