I take it you have a Brisbane model since your core speed is at 2.1 GHz. Go into your BIOS and lower your ram speed a step, so if it's at DDR2 800 set it at DDR2 667. Also lower your Hyper Transport speed one step, so if it's at 1GHz lower it to 800, which would lower the multiplier for the HT link to 4 instead of 5 and you would get a 1GHz speed when you got the FSB up to 250. Raise your core voltage by 0.1v and you shouldn't raise it more than that if you're using the stock cooler. Now Raise your FSB 10MHz at a time and run Prime 95 when you boot to make sure it's stable at those settings. You may also want to run Everest to make sure that your temps don't go to high. If you have the G2 stepping, then you shouldn't have a problem getting it past 2.6GHz. If you have a G1 stepping, which is more likely, then you'll probably be able to get around 2.6GHz but perhaps not too much further.