For single track I would be looking at a purpose built bike like a KTM or GasGas,it still depends if you like 2 or 4 stroke.Personally I went 2stroke as its a little less maintenance(no valves to check,not as many oil changes)plus on a KTM you can adjust the power valve for your performance preference.A Yamaha YZ250 also makes a good trail bike with minimum effort and should be cheaper than a KTM.The good thing about the Yamaha is the desighn has'nt changed much since 03,mostly only the suspension got a little better every year.Ross Rocher has some very good deals on 2011 GasGas bikes.