Here's what you do to find that out:
First, find out how high of an HT base clock your motherboard supports without overclocking your CPU core, northbridge/L3, HT bus, or RAM:
0. Your stock speeds are 2.60 GHz for the CPU (200 MHz x 13), your RAM runs at 333 MHz (200 MHz x 1.67), your HT bus and L3 run at 2.00 GHz (200 MHz x 10). Adjust the relevant multipliers to keep the CPU, RAM, HT bus, and northbridge/L3 at those speeds at all times.
1. Adjust the HT base clock up by 10 MHz and run a program that will stress the computer. Use something like Orthos to check the stability for 10 minutes or so. If it runs for 10 minutes, bump the HT base clock 10 MHz again and repeat, dropping multipliers to keep the RAM, CPU, and HT bus clocks at or below their stock clocks.
2. Watch the temperature of the northbridge if you have a thermal sensor. Try to keep it under 80 C. If you exceed that, you've hit as high of a northbridge clock as you are going to tolerate unless you put a better heatsink on the northbridge.
3. When the system is not stable any more (Orthos fails, you get BSODs, etc.) you can either say the HT clock is good enough (280-290 MHz will be enough) or you can give more volts to the northbridge. I wouldn't go more than 10-15% over stock voltage.
4. When you decide to stop, write down the maximum HT clock you get.
Next, you need to figure out how fast your RAM can run:
0. Adjust the multipliers to keep the CPU and HT bus speed at or below stock speeds.
1. Keep the memory divider at 1.67 ("DDR2-667 speed") and bump the HT base clock up 5 MHz. Run Memtest86+ from a bootable CD to assess RAM stability. If it passes a round, keep bumping the HT base clock up by 5 MHz at a time until the RAM no longer passes Memtest86+.
2. When the RAM fails Memtest86+, you can either slacken the timings (e.g. going from 4-4-4-12 to 5-5-5-15) and/or increasing RAM voltage if you feel the RAM is not going as fast as you want, otherwise you can stop.
Finally, let's overclock that CPU!
0. Keep your maximum HT clock and RAM clocks in mind and adjust the multipliers so you do not exceed them. Also, keep the HT bus multiplier low enough that the HT bus clock does not exceed 2.00 GHz.
1. Increase the HT bus speed by 10 MHz and run Orthos for 10 minutes or so. Make sure your CPU temps don't go over 60 C. Keep bumping up the HT bus clock until your CPU does not pass Orthos any more.
2. Increase the Vcore by a 0.05 V and then retry running Orthos at that speed when your CPU fails Orthos or you get a lockup or a BSOD. Keep going between this step and #1 until your temps reach 60 C (stop) or you are running 1.45 V to the CPU (stop.)
Now you have found your maximum overclock. This is a tedious process that takes a day or so to find, but you will find out what the limits of your system are. A Phenom II can run at close to 4.0 GHz on a good board, so you have some room left or a subpar component somewhere.