OCNoob's got you pretty much covered here, he's gonna have to change his name soon to OCModerate.
Looking at the specs for your board, you do have the option of an AGP/PCI lock. That's going to be critical, as leaving unlocked will cause your graphics card and any PCI peripherals to be OC'd as well. Specifically on your board, under the 'Configure System Frequency/Voltage' heading, make sure 'Asynchronous Frequency' is disabled.
My advice is to manually OC your system, don't let Asus handle it. I'm not a fan of automatic overclocking. By tweaking your settings yourself, you'll have a much better chance at success and stability. Under that same category above, set your 'AI Overclock Tuner' to manual and adjust your 'CPU Frequency' appropriately. Setting it at 200 (400fsb effective) will yield you 3200+ speeds, though it may take some further tweaking to get it stable at that speed. It's also going to be highly dependent on the quality of your RAM.
I'm using Corsair Value select (2X256MB). Nice litttle dual channel modules they are. I just tried to OC this thing by 20% using Asus' standard BIOS tweak, and I was met with the same system instability I encountered when I tried this the first time. I didn't see the AI overclock tuner setting in the BIOS, but I did see "CPU multiplier" (I think that's what it was). Is there something else I'm missing?