2600mhz = 200FSB x 13 multiplier
1. Enable AGP/PCI fix/lock in the BIOS
2. PC2700 = 333mhz; so to enable ratios :
For instance if you overclocked the FSB to 240, your cpu speed is now 13 x 240 = 3120
You have the option of going with 5:4, 3:2 or 1:1 ratio. 1:1 would mean that your memory must run at 240/1*1 = 240mhz(480 effective), 5:4 = 240/5*4 = 192mhz (384) - overclock; 3:2 = 240/3*2 = 160 (320 effective) - slightly below spec. Without overclocking the RAM, your highest overclock can be 249FSB or (3237mhz). I think your motherboard has other ratios as well though so see which one gives the highest memory speed at your overclocking limit.
3. There are various ways to to adjust voltage. The safe P4 voltage is around 1.65 (besides anything above that would most likely require watercooling at least). Your first option could be to adjust the FSB in increments of 10 starting with 200 and adjust the ratios accordingly (you could keep it at 3:2 right off the bat) until the cpu either breezes, can no longer boot, or you get a blue screen of death. At this point you should increase the voltage slightly and try again until you have supplied enough voltage. Then start adjusting the FSB again and applying the voltage until needed. Your p4 2.6 should easily reach 3.2ghz on 1.6V. If you motherboard has other ratios that allow you to keep increasing the FSB without overclocking the memory then you could utilize that as well.
Your other option could be to increase the cpu voltage to 1.65 right away and just adjust the FSB until your cpu freezes, locks up, etc. Then back down slightly to find a stable overclock. This is somewhat faster because you wont have to adjust the voltage every few increments of FSB as the case above.
My personal recommendation would be to see how far you can push your ram before you start overclocking the cpu. You should first adjust the FSB down to 166 so your ram is in synchronous mode with the cpu. Increase the DDR voltage to 2.8 and see how far you can increase the speed on the ram. This will tell you if you can do PC3200 or higher speeds. If this is possible and you do get 250FSB, you should enable 5:4 ratio so your RAM runs at the fastest possible speed (400 in this case). Of course lower latency is just as important as raw bandwidth for P4, if not more. If you have to losen the RAM timings too much to overclock, then it might be better to run slightly lower memory speed with tighter timings. You could always do benchmarks though once you reach your final overclock.
Make sure to download the latest bios from Asus' website before overclocking.
Download CPU-Z to see RAM speed and timings. You could also download WCPUID3.3 to see FSB, cpu speed, voltage. Finally Sisoftware Sandra 2004 could be used to see CPU temperature.
You can also use Asus PC Probe (i believe that's the name of the hardware monitoring program for your company) to oversee voltage. This is important because oftentimes motherboards can undervolt or overvolt. You do not want to have 1.68V when you select 1.65 in the BIOS, or you will risk a short-lived P4.
Hope this helps. Good luck.