the FSB controls the ratio between the speed of the RAM and processor. So if you have DDR400 RAM, you want the ram to run at 400mhz, so you set the FSB at 200 (DDR = Double Data Rate, so 2x200 =400). So since the FSB is at 200, you change the multiplier to whatever setting works for you. Since the 2600+ is stock speed at 2 gigs, you want to start the multiplier at 10. So 10x200 = 2000 mhz = 2 gigahertz. Then you up the multiplier bit by bit and see if your processor can handle it. If it doesnt stay stable, try upping the Vcore (the voltage to you CPU) and that will stablize the processor. Each time you up the multiplier, it is a good idea to stress your CPU to find it's max temperature, so get something Like Motherboard Monitor to watch the temp, and download Prime 95 to test it. You probably want to keep your Athlon XP CPU temps below 60C. So overclock as much as you can without the temps going to high.
So to really answer your question, you need to tell us what speed you want to run at, because "400 mhz FSB" really has nothing to do with your CPU as much as your RAM. If you have DDR333, it will be pretty hard to get your FSB to 400 megahertz. So make sure you have DDR400 RAM.
And if you are asking what cooling will give you the highest overclock, gotta be water cooling. I recommend the Thermaltake Bigwater set, as it can be found for under 150 (seach pricegrabber), and is one of the best Liquid cooling solution kits out there. If you dont want to shell out too much money for cooling, thats fine. Thermaltake also makes some of the best air hsf's. The Volcano 12+, or if you dont like fan's loud, go for the Silent Performance one. Zalman makes some good fan's too, but i dont have much experience with them. Someone else can probably tell you what is good and what isnt.
If you need to know more, we need to know more too. Tell us what kind of RAM and motherboard you have, and that will help us explain stuff to you much easier.