The Abit ST6 is a great mobo with lots of overclocking features that is fairly reasonable and the nice thing about it is that you can upgrade to the 1.2 gig Tualatin later on when you can afford it (they often overclock up to the 1.4 to 1.5 gig range). If you're on an extremely tight budget, then you might need to look around for an used motherboard.
Your current Celeron 600 is a cB0 stepping, not as good of an overclocker as the cC0 C-600 CPUs, but still a classic overclocking friendly chip. With a good CPU cooler and good thermal paste (and maybe an extra case fan or two), you should expect on average, up to 900 to 950 MHz.