I know of a program, called Easy Tune, I'm sure it works with Gigabyte boards, because, the proggie was made by Gigabyte. Not sure if it can work with other mobo's, but this is basically a CPU overclocking tool for begginers, but, it has advanced options.
Are you sure it runs at 66? If so your only using 1/2 of your processor power....which means a 700 or so mhz processor?
The speed of a processor is the FSBxCPU Mulitplier. Which in your case should be 133x11, the FSB can be changed in the bios of most computers(Except the K7S5A and some others) and the multiplier can only be changed by "Unlocking" the cpu, which means modding the cpu. Look around the net for complete instructions on how to do that.
you can overclock a k7s5a. Ive built two machines with those boards and got a 1.2 duron up to 1.4 gig with it. Theres a beta bios floating around i think its be cheeba but im not sure.. Use ddr and good cooling and youll be very happy with what the board can do. This may sound stupid... but ecs makes an agp video card based on the crappy sis 305 chip.. but when you plug both of them together it doubles the speed you boot. So if your not really into graphics but wanna go fast... might wanna check it out