1999 Honda Accord Head Gasket

I have a 99 accord 4-cyl.125K, idle rough at low end in gear and gets sluggish above 4000RPM. I have oil in the radiator and overflow and dripping below the car (no normal oil leaking spots) went through 3+ quarts oil over 3000 miles/3 weeks. Oil change guy says it seems to be head gasket, does not seem to be any water or other fluid in the engine when I check to the dipstick. I am getting no smoke or smell and the radiator is more than full. Is it the head gasket?? is it tough to change on a honda 4cyl?? estimate cost???