Chances are your 2.6 will top out at 3.5ghz unless you use watercooling.
Since 2600 = 13 multiplier x 200FSB = 2600mhz, that would mean 3500 is achieved at around 270FSB. Since PC4000 cannot attain 540mhz unless you overclock it, you'll have to either get PC4200 or 4400. Of course it's not guaranteed that 2.6 will hit 3.5.
Regardless, there are ratios like 5:4 and 3:2 and 1:1. So your best bet would be to use the highest FSB first of all and the fastest memory speed at the lowest possible timings possible. Therefore, 270FSB at 5:4 ratio => 270FSB:216mhz memory. 216mhz memory x 2 = 433mhz or PC3500. Therefore, PC3500 at 2-3-3-6 or tighter timings is just as good as 270FSB 1:1 at 3-4-4-8 PC 4400 offers. Check out Muskin Level 1 2-3-3 3500 its fairly inexpensive on Newegg at around $60-65 a piece and its what you need unless you want to go Mushkin Level 2 (2-2-2-5)
My recommended heatsink for the platform is Zalman CNPS7000A-ALCu (or without the A as that designation supports socket 462 which you dont care about i bet). Also you could consider SLK-900/800 with a panaflo 12V fan or a smart fun but that might be more expensive and not offer much better temperatures. You'd want to replace the stock cooler if you intend to exceed 3.0-3.2ghz.
To overclock isolate cpu and motherboard from memory by either setting a ratio for memory and/or relaxing the memory timings. Then overclock the cpu as far as you can in 10mhz increments from 200 - 210 - 220, etc....until you hit about 250 you might start to have problems, increase the voltage from 1.48-1.55 (actual/bios) to 1.65 actual (at most I would say). Then see how far you can go. Use Prime95 to test the stability of CPU. Then adjust the ram ratio from 3:2 to 5:4 if you wish to increase Memory speed, and lower the timings to as tight as possible, adjust the ram to 2.7-2.8volts (some allow higher).
For additional cooling, I would have at least 1 intake fan at the front of the case, and an exhaust fan at the back, possibly a side intake and an exhaust on the top of the case as well if you have whole there....
With a decent power supply you shouldnt have a problem. The only thing, you should get the latest bios for the Sis chipset as the asus seems to have problems overclocking past 267fsb with the old bios. You can check the overclocking review in detail here:
Asus P4S800-E overclocked at Anandtech
you can read the whole article if you wish.
Good luck. Remember, a decent power supply is a necessity for a stable overclock and dont forget that.