well there is no set or guarenteed speed you can OC by adding a fan. You should take into consideration the average temperature in the room with the Compy in it. for instance, in my room, it is hella hot here, so adding a fan is basically useless, so i could probably get a good 10mhz before i run into problems. But i am soon looking to get a waterblock, and see how that holds up against the heat. but basically it is a trial and error process.
It all depends on the card. A Voodoo3 2000 will overclock to about 150-155 without cooling, a simple fan will let you get an extra 5-10Mhz and a good fan setup will let you get 10-15Mhz. I have memory heatsinks and a sheetmetal fram with a Socket7 fan attached and a 486 heatsink and fan on the back and run at near room temp at 166Mhz. I have gotten to 183Mhz befor it started having trouble, but ran very hot. The GeForce 256 usually hit 150/350 and the GeForce 2 GTS hits 240/360. The TNT2 could hit about 185/210 with good cooling.
Some extreme people have used peltiers and water cooling to get a Voodoo3 3000 to 230MHz or a GeForce 256 DDR to 170/380 (which would probably beat the snot out of a GeForce2 GTS).