Carpetride,
YOu will need to get all the top soil up. You will see they color of the soil change 4-6 inches down and the top layer should go in a pile for a garden someday, or you may be able to sell it or at least give it away if you don't want to keep it around.
I would try to slice the turf around the perimeter of what you want to remove so you don't tear up what you want to stay. You can rip sod rippers for this purpose and then roll it up neatly by hand or you can pick it up with your FEL not so neatly but a good intro to FEL 101.
NOw I would try to attach a shank of some kind (and old leaf spring would work well) either to the bucket or to a rear implement to rip the perimeter, but that is just me...for best results rent a sod ripper, or walk around the perimeter with a shovel and slice it that way if the area isn't humoungous. If you have a sense of humor and patience to deal with the mess, just spray paint out what you want to remove and start tearing it up with the FEL. Go slow and read the soils color changes, you dont want to remove anything more than the topsoil as below that is a nicely compacted base that you will have to replace with compacteds gravel if you tear it up also. Throwing base in with your topsoil also minimizes its usefullness
You should know that FEL's are not really intended for digging in this manner, but rather moving already dug piles, But if you move slowly in low range with the bucket angled down slightly (and proably your foot on the axle lock) you will do what millions have done before you with few ill effects.
Good Luck!