Amd makes a really nifty, comprehensive piece of performance software called AOD. It can be had here:
http://sites.amd.com/us/game/downloads/amd-overdrive/Pages/overview.aspx
Most x2 2xx models will OC to between 3.5-3.8Ghz depending on the motherboard you are using. You should list the rest of your components so we know what we are dealing with, including the PSU you are using. assuming you have quality components, you have the advantage of many others OC'ing this model before you. using AOD is nice as you can shorten the time to OC before making the final stable settings semi permanent in the bios if you choose, or setup an OC profile in AOD to use only during intense operations or gaming if you like. If you want, you can jump right to a setting of 3.4Ghz by raising the HT ref to 242 and the V-core to 1.475 and running the built in stability test for 15 minutes. from there raise the HT ref by 5-10 Mhz and run the stability test until it breaks. You can then raise the Vcore in intervals of .025-.05 until stable. Remember your CPU has a locked multiplier so you will be OC'ing the whole system when you raise the HT ref clock (memory,Northbridge, etc) you may want to drop the HT/NB/CPU multiplier back while you find the maximum OC to eliminate it as a possible limiting factor. a couple things to keep in mind...
1) watch temperatures. Its a REALLY good idea to have after-market cooling (heatsink/Fan) while OC'ing
2) make sure you have adequate PSU, both in capacity and quality.
3) the maximum Vcore for this generation of CPU's is 1.55v, better yet, stay at or under 1.5v -1.525v
4) with very good cooling (and if you want to push up over 1.5 V) you may get over 4.0Ghz.
Here is TechPowerup's Oc of an x2 240, you can get an idea of the settings they used to get 3.8Ghz OC (typical OC for this CPU if you have decent MB,PSU etc)
Here is a look at the control screen of the AOD