Looks like you have a nice system. Oh yeah, you should be able to get a nice O/C out of that beast.
Enter the BIOS by pressing the "delete" key when the system first boots up. Click on the "uGuru Utility" on the BIOS menu page. This is where you'll do the overclocking. RAM timings are on another menu, but don't worry about that right now.
Set the "CPU Operating Speed" to "User Define" which allows manual adjustment to all the options below.
"External Clock" is the FSB or CPU frequency. Default for an E6400 is 267MHz, but the Abit BIOS usually sets 272MHz for a factory overclock.
"Multiplier Factor" should already be set to "X 8" which is default, just leave that alone.
"Estimated New CPU Clock" is grayed out and can't be adjusted. It just shows you the estimated CPU clock speed for your current settings.
Set the "N/B Strap CPU As" to "PSB1066" for Core 2 Duo processors.
For now, set the "DRAM Spec. (CPU : DRAM)" to "DDR2 533 (1:1)" This is how you set the memory ratio. Setting this to 1:1 for now will make sure the RAM isn't holding you back when you try to find your system limits.
"Estimated New DRAM Clock" is grayed out and can't be adjusted. It just shows you the estimated memory speed for your current settings.
Set the "Voltages Control" to "User define" which allows manual voltage adjustments to the options below.
Set the "CPU Core Voltage" to "1.350 V" which should get you a long ways when you start overclocking. Normally I tell people to leave this at default at first, but since you are watercooled, it should be ok.
Set the "DDR SDRAM Voltage" to 1.90 V" or "2.00 V" which will prevent boot problems due to RAM.
Set the "MCH & PCIe 1.5V Voltage" to "1.55 V" which will assure you can do the FSB that will get you to 3 gig.
FINALLY, you are ready to start overclocking. Set the "External Clock" to "280MHz" and then press "F10" to save the BIOS settings. This should overclock your system to 2.24 gig. Once you're in Windows, load the system to test stability and check temps in uGuru.
Congrats, you are overclocked. If everything is ok, go back into the BIOS and try another 5 MHz increment of FSB. Save the BIOS settings, start Windows, test for stability, and check temps. Repeat.