That's a pretty decent clock for 2.8g. Are you Stepping up the FSB to OC it? The BE CPU's have an Unlocked CPU Multiplier. You don't need to mess with the FSB Settings to OC it. Just step up your Mult.
If I remember correctly, you can juice that up to 3.0gigs You're at 2.8 now. Look up the program Prime95 and download it.. It's a Stability/Torture Test. Run that after you get a clock that you think is stable.. If Prime Runs for a couple of hours, you've got a good STABLE clock..
If not, you need to dial back some of your settings, or add a little voltage (Vcore).
Up the Multiplier 1x at a time.. 10x, Reboot, 11x Reboot, 12x Reboot.... etc etc... Until it does not boot windows. Then go back into your BIOS and step that Multiplier back a little bit.. Once it does not boot windows, then you need to up your Voltage .125v or .250 volts at a time. so, step the Multiplier up first things first.
Try a 12x Multiplier and see what that gets you.. Try to see what you can get without touching the voltage, If you want. I did that with my PII 940 and I got a Prime Stable 3.5gigs.
Hope that helps.. That's the jist of it soo far, without getting into other settings. I'm still learning and I don't know a LOT about OCing, but I've been getting a lot from here...
Here's a basic Video that helped me understand exactly what you needed to start with, as far as basic overclocking and it helped clear up the Acronyms and terminology for me.
YouTube - Overclocking (really basic)
-EDIT-
I just found a REALLY good thread! Explains everything in detail! I'm reading it now.
Dolk's Guide to the Phenom II - Overclockers Forums