My X2 3800+ reached upto 2.5Ghz stable and my X2 4400+ reached 2.5Ghz. You can see the difference, 3800 can overclock better than 4400. But dont go straight to 2.5 :P, just do it bit by bit.
To test the stability of your OCing download Prime95 or Orthos. Run it for at least 24 hrs.