I don't know what you have now, but here are three possibilities.
1) ECS P6S5AT $50 shipped, Celeron 1.0a (tualatin) $65 shipped, 256MB PC133 $43 shipped, total $158. This board will allow an easy
overclock to 1333MHz for the cpu, but future upgrades will not be possible, socket 370 memory bandwidth not so good.
2) ECS K7S5A $53 shipped, XP 1600 $93 shipped, 256MB generic PC2100 $59 shipped, total $205. Not much in overclocking options.
3) ECS K7S6A $77 shipped, Morgan 1.2GHz $59 shipped, Micron PC2100 $71 shipped, total $207. Can overclock the cpu to 1300MHz,
can overclock the fsb also for better DDR performance.
The second choice is the fastest set up, but I would go with choice three and get a better cpu later.