You won't get it to 2.8... no way no how.
But you may be able to hit 2 GHz, here's some tips to help you get there:
1. Cooling!!! Its vital that you have proper cooling. Check current CPU tempuratures; they should be in the ~45-50C neighborhood at most, since overclocking will raise the temps.
2. When overclocking, *always* adjust the multiplier FIRST before adjusting the FSB. Sometimes multipliers on Athlon CPUs are locked, so this may not be entirely possible, but its worth a try. If you adjust the multiplier and the computer no longer turns on, just reset the CMOS and it will adjust the BIOS to the factory settings.
3. You can also adjust the VCORE, or power to the CPU and get a little extra overclock, but this is risky and you really should know what you are doing to mess with this. Raising the VCORE also raises heat exponentially, so this is when your cooling system becomes especially important.
As always you have to accept the risk associated with overclocking; you are running an electronic component at a frequency higher than it was designed for. Some CPUs are more robust to overclocking than others, so your mileage may vary, Good Luck!