you'll do something called front side bus (FSB) or cpu frequency overclocking. to start you need to turn off LLC, quiet and cool, turn the advanced cpu settings from auto to manual, go to the ram frequency and set it as low and slow as you can. turn your north bridge frequency down as low as it will go, and do the same for your hyper transport frequency.
then you're going to go back to the cpu frequency. now i don't know your motherboard so some of these things might be under different names or not present. but the cpu frequency/fsb should be there. there should also be a cpu multiplier. now understand since your cpu is locked the multiplier may only work going lower (not higher) or it may not work at all... it depends on the motherboard. this is a lot easier if it atleast works going backwards but we can work around it if you don't have control over it at all.
Assuming you can set it lower... lower the cpu multiplier by -0.5 (i think stock is x14, so you'd be dropping it to x13.5), and raise the cpu frequency by 7, to 207. this will keep your cpu's clock speed to pretty close to exactly where you started (207x13.5=2.8ghz). now save and restart. make sure your computer loads into windows. then restart, and go back into the bios. lower the multiplier by another -0.5 and raise the cpu frequency up to 215 (215x13=2.8ghz). save and load into windows. keep doing this until the computer fails to POST or fails to load into windows. When that happens you know you've hit your cpu's limits for FSB overclocking.
if it fails to post, don't panic, after 3 such failures your bios will reset itself, go back into the bios and set all the settings to the last good setup (the last time it loaded into windows.) write down the cpu frequency/fsb number, and set it back to 200, then set the cpu multiplier back to 14.
now we're going to overclock. we know the max fsb your cpu can handle, so we have the max possible overclock... the rest is up to your system, power supply, cpu cooler and motherboard to see if it can handle it. (example, if your fsb topped out at 231, your max overclock would be ~3.2ghz 231*14=3,234 or 3.2ghz and change)
Now if your motherboard doesn't support using the multiplier like we just did, you're going to start here... you're just going to be flying blind as to where the limits will be, so you'll probably flail around a bit more when you hit that wall, trying to find a way around it.
what you'll do next is you'll raise your fsb by +10, save and restart the computer, see if you load into windows. if you do, restart and get into your bios. now you'll bump your fsb by +5, and save, load into windows, rinse and repeat (only add it in +5 increments), at some point you'll be unable to load into windows. what you'll do then is load into the bios, and add vcore. add +0.0125V to the base vcore (if you do not have a base vcore displayed in that system set it to 1.4v, that's a safe number to start at). Try to load into windows. Assuming you can load into windows what you do next will depend on if you were able to discover your max fsb or not.
If you did discover it, and you're not there yet, you can load back into your bios and bump your fsb safely again. If you did not discover it, you will have to stabilize at this clock speed first. Get prime95, get hwmonitor. keep an eye on temps, while you run prime95 for a few passes. if prime crashes, a core fails, windows freezes, the computer black screen restarts your vcore is still too low. if you get a BSOD it could still be an undervolted cpu... though typically this is a ram error, and what you'll be looking at doing is adding a little to your cpu/northbridge voltage.
So depending on the crash you'll add a little more 0.0125V to either the vcore or cpu/nb. If your temps get up over 58C and you crash that's a temp cause failure, and you either need a better cpu cooler or lower overclock with lower voltages.
Now then, assuming everything is kosher and you were able to stabilize at that clock speed or you pressed on without bothering to, you'll keep on bumping the cpu frequency by 5, and when windows fails to load bumping the vcore a little till it loads. eventually you'll hit the fsb max, at which point you'll need to stop and try to stabilize like i just outlined. make sure you don't overheat, you're going to need some solid voltage increases (likely) to get it stable, which means some serious heat. if you can get it to run prime95 for a solid 8 hours or so without overheating or crashing, then your overclock is stable and you're good to go.
The next thing you'll do is bring back up your ram, north bridge and hyper transport speeds. try to bring them as close to stock as you can, PhIIs love their ram around 1333 in speed, so don't go higher if you can avoid it, the nb and ht should be close to 2000 too... (it won't be even, by changing the fsb/cpu frequency, you've changed the base frequency for the ram, nb, and ht as well)